chore: make it easier to pin Rust versions in CI (#4448)

When backporting patches to LTS branches, we often run into CI failures due to
changes in rust. Newer rust versions add more lints, which break CI. We really
don't want to also have to backport patches that fix CI, so instead, LTS branches
should pin the stable rust version in CI (e.g. #4434).

This PR restructures the CI config files to make it a bit easier to set a specific rust
version in CI.

.github/workflows/loom.yml

@@ -10,6 +10,8 @@ name: Loom
 env:
   RUSTFLAGS: -Dwarnings
   RUST_BACKTRACE: 1
+  # Change to specific Rust release to pin
+  rust_stable: stable
 
 jobs:
   loom:
@@ -28,8 +30,11 @@ jobs:
           - time::driver
     steps:
       - uses: actions/checkout@v2
-      - name: Install Rust
-        run: rustup update stable
+      - name: Install Rust ${{ env.rust_stable }}
+        uses: actions-rs/toolchain@v1
+        with:
+            toolchain: ${{ env.rust_stable }}
+            override: true
       - uses: Swatinem/rust-cache@v1
       - name: loom ${{ matrix.scope }}
         run: cargo test --lib --release --features full -- --nocapture $SCOPE