Reduce 3rd party license check frequency

Changed triggers of CI job to run only on changes to the
Cargo.* files. This reduces the frequency of the 3rd party license
check to the required minimum, which should help with not running
into 429 errors when querying Clearly Defined.

Author: sophokles73

.github/workflows/check-dependencies.yaml

@@ -14,8 +14,6 @@
 name: Check 3rd party dependencies
 
 on:
-  push:
-    branches: [ main ]
   pull_request:
     paths:
     - "Cargo.*"