ci: fix github action syntax for variables (#2998)

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`${ runner.os }` -> `${{ runner.os }}`

## Checks
- [x] I am happy for maintainers to push small adjustments to this PR,
to speed up the review cycle
- [x] I have checked that there are no existing pull requests for the
same thing

.github/workflows/rust.yml

@@ -110,7 +110,7 @@ jobs:
             ~/.cargo/registry
              ~/.cargo/git
              target
-          key: ${ runner.os }-cargo-debug-${{ hashFiles('**/Cargo.lock') }}
+          key: ${{ runner.os }}-cargo-debug-${{ hashFiles('**/Cargo.lock') }}
 
       - name: Run cargo test
         run: cargo nextest run --lib --bins
@@ -135,7 +135,7 @@ jobs:
             ~/.cargo/registry
              ~/.cargo/git
              target
-          key: ${ runner.os }-cargo-debug-${{ hashFiles('**/Cargo.lock') }}
+          key: ${{ runner.os }}-cargo-debug-${{ hashFiles('**/Cargo.lock') }}
 
       - name: Run cargo check (all features)
         run: cargo check --all-features --workspace
@@ -184,7 +184,7 @@ jobs:
             ~/.cargo/registry
              ~/.cargo/git
              target
-          key: ${ runner.os }-cargo-debug-${{ hashFiles('**/Cargo.lock') }}
+          key: ${{ runner.os }}-cargo-debug-${{ hashFiles('**/Cargo.lock') }}
 
       - name: Run cargo test
         run: cargo nextest run --test '*'