ci/check-shell: only run if shell.nix or ./ci/** is changed

saves a bit of CI time

Author: Mic92

.github/workflows/check-shell.yml

@@ -2,6 +2,9 @@ name: "Check shell"
 
 on:
   pull_request_target:
+    paths:
+      - 'shell.nix'
+      - './ci/**'
 
 permissions: {}