ci: remove uses of unmaintained actions-rs actions (#5316)

- Use dtolnay/rust-toolchain instead of actions-rs/toolchain
- Use cargo/cross directly instead of actions-rs/cargo
- Use rustsec/audit-check instead of actions-rs/audit-check

.github/workflows/loom.yml

@@ -34,10 +34,9 @@ jobs:
     steps:
       - uses: actions/checkout@v3
       - name: Install Rust ${{ env.rust_stable }}
-        uses: actions-rs/toolchain@v1
+        uses: dtolnay/rust-toolchain@master
         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