Pin to an old compiler for linting on CI

1.53.0 is to be released tomorrow, and currently has a clippy regression
that causes an ICE.

https://internals.rust-lang.org/t/1-53-0-prerelease-testing/14884/2?u=jtgeibel

Author: jtgeibel

.github/workflows/ci.yml

@@ -67,6 +67,7 @@ jobs:
   backend-lint:
     name: Backend (linting)
     runs-on: ubuntu-20.04
+    rust: 1.52.1 # Pin to older version until a clippy regression is fixed
 
     env:
       CARGO_INCREMENTAL: 0