Fix Julia cache flood in CI workflows (#14410)

* Bump julia-actions/cache to v3

v2 is a composite action whose post-save and delete-old-caches steps
don't run reliably, letting per-run julia caches (~250 MB each)
accumulate on refs/heads/main until the repository hits its 10 GB
cache cap. v3 is a JavaScript rewrite with a proper post hook. It also
uses Node.js 24 directly, dropping the latent Node 20 exposure from
v2's transitive actions/cache dependency.

* Fix cleanup-caches workflow for fork PRs

pull_request events from forks ship a read-only GITHUB_TOKEN regardless
of the permissions: block, so gh cache delete fails with HTTP 403 and
the fork PR's ~1-2 GB of caches leak into the repo's 10 GB cache budget.
Observed on PR #14374 (8/8 deletes returned 403, run succeeded because
set +e swallowed the failures).

pull_request_target runs in the base-branch context with full write
permissions. Safe for this workflow: no PR code is checked out, the
steps only call gh cache list/delete.

Author: cderv

.github/workflows/cleanup-caches.yml

@@ -1,6 +1,10 @@
 name: Cleanup github runner caches on closed pull requests
 on:
-  pull_request:
+  # pull_request_target is required so fork PRs can delete their own caches on close.
+  # pull_request from a fork ships a read-only GITHUB_TOKEN regardless of the
+  # permissions: block, causing gh cache delete to fail with HTTP 403.
+  # Safe here: no PR code is checked out, the workflow only calls gh cache list/delete.
+  pull_request_target:
     types:
       - closed
 

.github/workflows/test-smokes.yml

@@ -223,7 +223,7 @@ jobs:
           version: "1.12.5"
 
       - name: Load Julia packages from cache
-        uses: julia-actions/cache@v2
+        uses: julia-actions/cache@v3
         with:
           cache-name: julia-cache;workflow=${{ github.workflow }};job=${{ github.job }};julia=${{ steps.setup-julia.outputs.julia-version }}