Only enforce clippy on stable

.github/workflows/tests.yml

@@ -59,7 +59,6 @@ jobs:
         run: |
           cargo --version
           rustc --version
-      # TODO: Maybe also check clippy and rustfmt here.
       - name: Build
         run: cargo build --all-targets
       - uses: taiki-e/install-action@v2
@@ -112,6 +111,9 @@ jobs:
       - name: Check rustfmt
         run: cargo fmt --all --check
       - name: Check clippy
+        if: matrix.version == 'stable'
+        # Clippy checks can vary between versions in a way that makes it a bit
+        # fiddly to satisfy them all, so only insist that they pass on stable.
         run: cargo clippy --all-targets --all-features -- -D warnings
 
   minimal-versions: