ci(olean_report): fetch PR oleans from fork's cache (leanprover-community#38894)

Fixes an issue noticed in [this comment](leanprover-community#38653 (comment)) where the olean report said that 5066 modules were removed from a PR that only added `to_dual` tags.

The issue is that the `olean_report` workflow only fetched oleans from the `leanprover-community/mathlib4` cache and not the fork cache. We fix this by also fetching the head repo name from `gh pr view --json headRepository` and re-running `cache get --repo=<head_repo>`, mirroring what `build_template.yml` already does for regular CI.

Prepared with Claude code.

Author: bryangingechen

.github/workflows/olean_report.yaml

@@ -69,11 +69,12 @@ jobs:
         run: |
           pr_json=$(gh pr view "${{ github.event.issue.number }}" \
             --repo "${{ github.repository }}" \
-            --json headRefOid,baseRefOid,baseRefName)
+            --json headRefOid,baseRefOid,baseRefName,headRepository)
           {
             echo "head_sha=$(echo "$pr_json" | jq -r '.headRefOid')"
             echo "base_sha=$(echo "$pr_json" | jq -r '.baseRefOid')"
             echo "base_ref=$(echo "$pr_json" | jq -r '.baseRefName')"
+            echo "head_repo=$(echo "$pr_json" | jq -r '.headRepository.nameWithOwner')"
           } >> "$GITHUB_OUTPUT"
 
       # Using refs/pull/N/head works for both same-repo and fork PRs, since
@@ -175,6 +176,9 @@ jobs:
         run: |
           cd pr-branch
           lake env "$CACHE_BIN" get
+          # Run again with --repo in case this is a fork PR: fork PRs upload
+          # their oleans to the fork's Azure container, not the upstream one.
+          lake env "$CACHE_BIN" --repo="${{ steps.pr_info.outputs.head_repo }}" get
 
       - name: Check PR head oleans
         id: pr_oleans_check