fix(release): restore executable bit on check-cachix-pin.sh and invoke via bash

PR #20 left check-cachix-pin.sh with mode 100644: commit 667ac07 set it
executable (100755) but the follow-up 37772a5 flipped it back. The Release
workflow runs the script directly, so the "Populate Nix Caches" job failed
with Permission denied.

Restore the executable bit, and invoke the script via `bash` so the job no
longer depends on the mode bit being preserved.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: RaoulSchaffranek

.github/scripts/check-cachix-pin.sh

@@ -71,7 +71,7 @@ jobs:
           export PATH="$(nix build github:runtimeverification/kup --no-link --print-out-paths)/bin:$PATH"
           kup publish k-framework-binary .#komet-node --keep-days 180
           # Cachix has not been responding to 'cachix pin' requests made under the hood by kup. Verify the push and pin manually.
-          .github/scripts/check-cachix-pin.sh
+          bash .github/scripts/check-cachix-pin.sh
 
       - name: 'On failure, delete drafted release'
         if: failure()