share pip_constraints between CI run steps

.github/workflows/pytests.yml

@@ -19,6 +19,7 @@ jobs:
     env:
       coverage-on-version: "3.9"
       use-mpi: True
+      pip_constraints: ${{ env.home }}/pip_constraints.txt
     defaults:
       run:
         shell: bash -l {0}
@@ -39,14 +40,13 @@ jobs:
 
       - name: Install dependencies from pip
         run: |
-          export PIP_CONSTRAINTS=$HOME/pip_constraints.txt
-          echo "numpy<2" > $PIP_CONSTRAINTS
-          python3 -m pip install -c $PIP_CONSTRAINTS wheel setuptools numpy scipy click matplotlib pyyaml spglib rdkit flake8 pytest pytest-cov requests
+          echo "numpy<2" >> ${{ env.pip_constraints }}
+          python3 -m pip install -c ${{ env.pip_constraints }} wheel setuptools numpy scipy click matplotlib pyyaml spglib rdkit flake8 pytest pytest-cov requests
 
       - name: Install latest ASE from pypi
         run: |
-          echo PIP_CONSTRAINTS $PIP_CONSTRAINTS
-          python3 -m pip install -c $PIP_CONSTRAINTS ase 
+          echo PIP_CONSTRAINTS ${{ env.pip_constraints }}
+          python3 -m pip install -c ${{ env.pip_constraints }} ase 
           echo -n "ASE VERSION "
           python3 -c "import ase; print(ase.__file__, ase.__version__)"
 
@@ -79,46 +79,46 @@ jobs:
 
             git clone https://github.com/phonopy/phonopy
             cd phonopy
-            python3 -m pip install -c $PIP_CONSTRAINTS -e . -vvv
+            python3 -m pip install -c ${{ env.pip_constraints }} -e . -vvv
             cd ..
 
             git clone https://github.com/phonopy/phono3py
             cd phono3py
-            python3 -m pip install -c $PIP_CONSTRAINTS -e . -vvv
+            python3 -m pip install -c ${{ env.pip_constraints }} -e . -vvv
             cd ..
 
       - name: Install Quippy from PyPI
-        run: python3 -m pip install -c $PIP_CONSTRAINTS quippy-ase
+        run: python3 -m pip install -c ${{ env.pip_constraints }} quippy-ase
 
       - name: Install xTB (before things that need pandas like MACE and wfl, since it will break pandas-numpy compatibility by downgrading numpy)
         run: |
           conda install -c conda-forge xtb-python
-          python3 -m pip install -c $PIP_CONSTRAINTS typing-extensions
+          python3 -m pip install -c ${{ env.pip_constraints }} typing-extensions
           # install pandas now to encourage compatible numpy version after conda regressed it
-          python3 -m pip install -c $PIP_CONSTRAINTS pandas
+          python3 -m pip install -c ${{ env.pip_constraints }} pandas
 
       - name: MACE
         run: |
           echo "search for torch version"
           set +o pipefail
-          torch_version=$( python3 -m pip install -c $PIP_CONSTRAINTS torch== 2>&1 | fgrep 'from versions' | sed -e 's/.* //' -e 's/)//' )
+          torch_version=$( python3 -m pip install -c ${{ env.pip_constraints }} torch== 2>&1 | fgrep 'from versions' | sed -e 's/.* //' -e 's/)//' )
           echo "found torch version $torch_version, installing cpu-only variant"
-          python3 -m pip install -c $PIP_CONSTRAINTS torch==${torch_version}+cpu -f https://download.pytorch.org/whl/torch_stable.html
+          python3 -m pip install -c ${{ env.pip_constraints }} torch==${torch_version}+cpu -f https://download.pytorch.org/whl/torch_stable.html
           echo "installing mace"
-          python3 -m pip install -c $PIP_CONSTRAINTS git+https://github.com/ACEsuit/mace.git@main
+          python3 -m pip install -c ${{ env.pip_constraints }} git+https://github.com/ACEsuit/mace.git@main
           python3 -c "import mace; print(mace.__file__)"
 
       - name: Julia and ace fit
         run: |
-          python3 -m pip install -c $PIP_CONSTRAINTS threadpoolctl
+          python3 -m pip install -c ${{ env.pip_constraints }} threadpoolctl
           wget https://julialang-s3.julialang.org/bin/linux/x64/1.8/julia-1.8.1-linux-x86_64.tar.gz
           tar xzf julia-1.8.1-linux-x86_64.tar.gz
           # note that this hardwires a particular compatible ACE1pack version
           echo 'using Pkg; pkg"registry add https://github.com/JuliaRegistries/General"; pkg"registry add https://github.com/JuliaMolSim/MolSim.git"; pkg"add [email protected], ACE1, JuLIP, IPFitting, ASE"' > ace1pack_install.jl
           ${PWD}/julia-1.8.1/bin/julia ace1pack_install.jl
 
       - name: Install wfl (expyre and universalSOAP are dependencies)
-        run: python3 -m pip install -c $PIP_CONSTRAINTS .
+        run: python3 -m pip install -c ${{ env.pip_constraints }} .
 
       - name: Install Quantum Espresso
         run: |
@@ -152,7 +152,7 @@ jobs:
         run: |
           # this can eaily be turned off if needed
           conda install -c conda-forge mpi4py openmpi pytest-mpi
-          python3 -m pip install -c $PIP_CONSTRAINTS mpipool
+          python3 -m pip install -c ${{ env.pip_constraints }} mpipool
 
       - name: Install and configure slurm and ExPyRe
         run: |