Fix PR lookup for fetchdep.sh (#10990)

Context: `fetchdep.sh` attempts to check out a github repository based on the
details in a pull request. To do this, it needs to know how to find the pull
request. So, the github workflows attempt to set environment variables to tell
it. Unfortunately, they currently disagree about what the names of the
environment variables should be.

This appears to have been introduced by #8498.

To simplify matters, we may as well have the script use `${GITHUB_REPOSITORY}`
directly, and remove te unused `REPOSITORY` env var from the workflows.

.github/workflows/tests.yml

@@ -20,11 +20,12 @@ on:
 concurrency:
     group: ${{ github.workflow }}-${{ github.ref }}-${{ github.event_name }}
     cancel-in-progress: true
+
 env:
     ENABLE_COVERAGE: ${{ github.event_name != 'merge_group' && inputs.disable_coverage != 'true' }}
-    # These must be set for fetchdep.sh to get the right branch
-    REPOSITORY: ${{ github.repository }}
+    # fetchdep.sh needs to know our PR number
     PR_NUMBER: ${{ github.event.pull_request.number }}
+
 jobs:
     jest:
         name: Jest