Pin clippy check to 1.49.0

It can be rather surprising when new lints pop up when a new stable
toolchain is released. Let's pin this check to a specific version to
avoid those surprises.

.github/workflows/clippy.yml

@@ -17,7 +17,7 @@ jobs:
       - uses: actions-rs/toolchain@v1
         with:
           profile: minimal
-          toolchain: stable
+          toolchain: 1.49.0
           override: true
           components: clippy
       - uses: actions-rs/clippy-check@v1