Install and cache Rust toolchain (#2353)

On GH-hosted runners, this isn't a problem. But on self-hosted runners
with persistent disks, we hit this issue:
https://github.com/Swatinem/rust-cache#known-issues

This PR should fix that by ensuring the Rust toolchain is installed. It
also installs the toolchain after the cache has been restored so we can
take advantage of any cached artifacts.

.github/workflows/kotlin.yml

@@ -32,14 +32,12 @@ jobs:
         working-directory: ./kotlin/android
     steps:
       - uses: actions/checkout@v4
-      - name: Update toolchain
-        run: rustup show
-        working-directory: ./rust
       - uses: Swatinem/rust-cache@v2
         with:
           workspaces: ./rust
           key: ubuntu-22.04-${{ runner.arch }}
           save-if: ${{ github.ref == 'refs/heads/main' }}
+      - uses: dtolnay/rust-toolchain@stable
       - uses: actions/setup-java@v3
         with:
           distribution: temurin

.github/workflows/rust.yml

@@ -27,13 +27,14 @@ jobs:
     runs-on: ${{ matrix.runs-on }}
     steps:
       - uses: actions/checkout@v4
-      - name: Update toolchain
-        run: rustup show
       - uses: Swatinem/rust-cache@v2
         with:
           workspaces: ./rust
           save-if: ${{ github.ref == 'refs/heads/main' }}
           key: ${{ matrix.runs-on }}-${{ runner.arch }}
+      - uses: dtolnay/rust-toolchain@stable
+        with:
+          components: rustfmt,clippy
       - run: cargo fmt -- --check
       - run: |
           cargo doc --all-features --no-deps --document-private-items ${{ matrix.packages }}
@@ -62,14 +63,13 @@ jobs:
     runs-on: ${{ matrix.runs-on }}
     steps:
       - uses: actions/checkout@v4
-      - name: Update toolchain
-        run: rustup show
       - uses: Swatinem/rust-cache@v2
         with:
           # Prevents runner.os caches from conflicting
           key: ${{ matrix.runs-on }}-${{ runner.arch }}
           workspaces: ./rust
           save-if: ${{ github.ref == 'refs/heads/main' }}
+      - uses: dtolnay/rust-toolchain@stable
       - run: cargo test --all-features ${{ matrix.packages }}
 
   smoke-test-relay:
@@ -79,11 +79,10 @@ jobs:
         working-directory: ./rust/relay
     steps:
       - uses: actions/checkout@v4
-      - name: Update toolchain
-        run: rustup show
       - uses: Swatinem/rust-cache@v2
         with:
           workspaces: ./rust
           key: ubuntu-22.04-${{ runner.arch }}
           save-if: ${{ github.ref == 'refs/heads/main' }}
+      - uses: dtolnay/rust-toolchain@stable
       - run: ./run_smoke_test.sh

.github/workflows/static-analysis.yml

@@ -7,13 +7,12 @@ jobs:
     runs-on: ubuntu-22.04
     steps:
       - uses: actions/checkout@v4
-      - name: Update toolchain
-        run: rustup show
       - uses: Swatinem/rust-cache@v2
         with:
           workspaces: ./rust
           key: ubuntu-22.04-${{ runner.arch }}
           save-if: ${{ github.ref == 'refs/heads/main' }}
+      - uses: dtolnay/rust-toolchain@stable
       - name: Check version is up to date
         run: |
           make version

.github/workflows/swift.yml

@@ -48,14 +48,12 @@ jobs:
         working-directory: ./swift/apple
     steps:
       - uses: actions/checkout@v4
-      - name: Update toolchain
-        run: rustup show
-        working-directory: ./rust
       - uses: Swatinem/rust-cache@v2
         with:
           workspaces: ./rust
           key: ${{ matrix.runs-on }}-${{ runner.arch }}
           save-if: ${{ github.ref == 'refs/heads/main' }}
+      - uses: dtolnay/rust-toolchain@stable
       - uses: actions/cache/restore@v3
         name: Restore Swift DerivedData Cache
         with: