Run lints against a precise version of the rust toolchain

This will allow PRs with failed CI just because they happened with
a newer version of rustfmt/clippy than last time the master branch
was checked.

Author: glandium

.github/workflows/ci.yml

@@ -24,6 +24,7 @@ jobs:
         uses: ./.github/actions/rust-toolchain
         with:
           components: ${{ matrix.component }}
+          toolchain: 1.50.0
 
       - name: clippy version
         run: cargo clippy --version