ci: Actually use toolchain

Somehow at some point, additional rust toolchains became available in
CI. Thus, it's no longer sufficient to install the matrix toolchain, but
actually override it for the build directory.

Author: Naseschwarz

.github/workflows/ci.yml

@@ -41,6 +41,9 @@ jobs:
         toolchain: ${{ matrix.rust }}
         components: clippy
 
+    - name: Override rust toolchain
+      run: rustup override set ${{ matrix.rust }}
+
     - name: Rustup Show
       run: rustup show