Replace unmaintained actions-rs/cargo action in CI workflow (#455)

Basically all of the `actions-rs/*` actions are unmaintained. See
<actions-rs/toolchain#216> for more
information. Due to their age they generate several warnings in
CI runs.

To get rid of those warnings the occurrences of `actions-rs/cargo`
are replaced by direct invocations of `cargo`.

Author: striezel

.github/workflows/workspace.yml

@@ -34,7 +34,4 @@ jobs:
           components: rustfmt
 
       - name: Run cargo fmt
-        uses: actions-rs/cargo@v1
-        with:
-          command: fmt
-          args: --all -- --check
+        run: cargo fmt --all -- --check