ci: fix conditional

Author: robjtede

.github/workflows/ci.yml

@@ -63,7 +63,7 @@ jobs:
         run: echo '${{steps.toolchain.outputs.rustup-version}}'
 
       - name: Check lack of toolchain input or file results in stable
-        if: !matrix.write-toolchain-file && matrix.rust == null
+        if: matrix.write-toolchain-file == false && matrix.rust == null
         shell: bash
         run: |-
           rustcv="$(rustc --version)"