ci: run clippy only on stable Rust channel

Sparked by the comments about nightly clippy breaking the build
too often (see <#151 (comment)>),
this change will limit clippy to builds with a stable Rust
version only.

Author: striezel

.github/workflows/build.yml

@@ -73,6 +73,7 @@ jobs:
         with:
           command: clippy
           args: --all -- -D warnings
+        if: matrix.rust == 'stable'
       - name: Build with docs stub
         if: "contains(matrix.os, 'ubuntu') && matrix.php == '8.1'"
         env: