ci/rust: Change MSRV to cargo check

No reason to codegen just to throw it away.  We could test here too,
but eh.

.github/workflows/rust.yml

@@ -47,8 +47,8 @@ jobs:
           default: true
       - name: Cache Dependencies
         uses: Swatinem/rust-cache@ce325b60658c1b38465c06cc965b79baf32c1e72
-      - name: cargo build
-        run: cargo build --features=${{ env['CARGO_PROJECT_FEATURES'] }}
+      - name: cargo check
+        run: cargo check --features=${{ env['CARGO_PROJECT_FEATURES'] }}
   build-no-features:
     runs-on: ubuntu-latest
     container: quay.io/coreos-assembler/fcos-buildroot:testing-devel