github actions: Use rustup to install toolchain

Replace the actions-rs/toolchain ci script with direct calls to
the `rustup` tool. This is part of the default github actions
runner image, so it's one less dependency we need to track in
the repo. The actions-rs suite has also had an unresponsive
upstream for some years.

.github/workflows/nostd.yml

@@ -25,9 +25,7 @@ jobs:
         uses: actions/checkout@8ade135a41bc03ea155e62e844d188df1ea18608 # v4.1.0
 
       - name: Toolchain
-        uses: dtolnay/rust-toolchain@1482605bfc5719782e1267fd0c0cc350fe7646b8 # v1
-        with:
-          toolchain: ${{matrix.rust}}
+        run: rustup default ${{matrix.rust}}
 
       - name: Cache
         uses: Swatinem/rust-cache@a95ba195448af2da9b00fb742d14ffaaf3c21f43 # v2.7.0

.github/workflows/tests.yml

@@ -25,12 +25,9 @@ jobs:
         uses: actions/checkout@8ade135a41bc03ea155e62e844d188df1ea18608 # v4.1.0
 
       - name: Toolchain
-        uses: actions-rs/toolchain@16499b5e05bf2e26879000db0c1d13f7e13fa3af # v1.0.7
-        with:
-          profile: minimal
-          components: clippy, rustfmt
-          toolchain: ${{matrix.rust}}
-          override: true
+        run: |
+          rustup default ${{matrix.rust}}
+          rustup component add clippy rustfmt
 
       - name: Cache
         uses: Swatinem/rust-cache@a95ba195448af2da9b00fb742d14ffaaf3c21f43 # v2.7.0