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/element-web.yaml

@@ -15,10 +15,11 @@ on:
 concurrency:
     group: ${{ github.workflow }}-${{ github.ref }}-${{ github.event_name }}
     cancel-in-progress: true
+
 env:
-    # 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:
     build:
         name: "Build Element-Web"

.github/workflows/static_analysis.yaml

@@ -10,10 +10,11 @@ on:
 concurrency:
     group: ${{ github.workflow }}-${{ github.ref }}-${{ github.event_name }}
     cancel-in-progress: true
+
 env:
-    # 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:
     ts_lint:
         name: "Typescript Syntax Check"

.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

scripts/fetchdep.sh

@@ -10,8 +10,15 @@ defbranch="$3"
 
 rm -r "$defrepo" || true
 
-PR_ORG=${PR_ORG:-"matrix-org"}
-PR_REPO=${PR_REPO:-"matrix-react-sdk"}
+# figure out where to look for pull requests:
+#   - We may have been told an explicit repo via the PR_ORG/PR_REPO/PR_NUMBER env vars
+#   - otherwise, check the $GITHUB_ env vars which are set by Github Actions
+#   - failing that, fall back to the matrix-org/matrix-react-sdk repo.
+#
+# in ether case, the PR_NUMBER variable must be set explicitly.
+default_org_repo=${GITHUB_REPOSITORY:-"matrix-org/matrix-react-sdk"}
+PR_ORG=${PR_ORG:-${default_org_repo%%/*}}
+PR_REPO=${PR_REPO:-${default_org_repo##*/}}
 
 # A function that clones a branch of a repo based on the org, repo and branch
 clone() {