Fix signed commits CI job

The job was previously ran on "pull_request_target". This runs the
job on the base branch (i.e. the `main` branch of the
 1password/shell-plugins repo). Since we want to know if commits
are signed on all the commits to be merged (i.e. also on the fork),
I have changed this job to run on the merge commit instead.

Relevant documentation:
https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#pull_request_target

Note that the usage example of https://github.com/1Password/check-signed-commits-action also recommends running on the "pull_reqeusts" event.

Author: SimonBarendse

.github/workflows/pr-check-signed-commits.yml

@@ -1,5 +1,5 @@
 name: Check signed commits in PR 
-on: pull_request_target
+on: pull_request
 
 jobs:
   build: