TileDB-SOMA-R: define CXX20FLAGS in ~/.R/Makevars

TileDB-SOMA recently migrated from C++17 to C++20,
so I updated CXX17FLAGS to CXX20FLAGS.

However, I observed that no matter if I exported
CXXFLAGS in the build step, the install step, or
both, the deprecation warnings were not suppressed.

I was only able to suppress the deprecation warnings
by following the example upstream and defining
CXX20FLAGS in ~/.R/Makevars. Only then were the flags
added to the call to g++.

https://github.com/single-cell-data/TileDB-SOMA/blob/cd1e4876100d32f0f6a494bb7ba8e07868b1885e/.github/workflows/r-ci.yml#L102

.github/workflows/linux.yml

@@ -487,13 +487,12 @@ jobs:
           install.packages(binary_tarball, repos = NULL)
       - name: Dependencies
         run: cd apis/r && tools/r-ci.sh install_all
+      - name: Remove deprecation warnings
+        run: mkdir ~/.R && echo "CXX20FLAGS=-Wno-deprecated-declarations -Wno-deprecated" > ~/.R/Makevars
       - name: Build
-        run: |
-          export CXX17FLAGS="-Wno-deprecated-declarations -Wno-deprecated"
-          cd apis/r && R CMD build --no-manual --no-build-vignettes .
+        run: cd apis/r && R CMD build --no-manual --no-build-vignettes .
       - name: Install
-        run:
-          cd apis/r && R CMD INSTALL --install-tests --build tiledbsoma_*.tar.gz
+        run: cd apis/r && R CMD INSTALL --install-tests --build tiledbsoma_*.tar.gz
       - name: Upload R binary tarball as artifact
         uses: actions/upload-artifact@v4
         with: