[ci] Enable workflow_dispatch events

.github/workflows/ci.yml

@@ -84,10 +84,15 @@ on:
 
   # Rebuild 'main' every week. This will account for changes to dependencies
   # and to Rust, either of which can trigger new failures. Rust releases are
-  # every 6 weeks, on a Thursday; this event runs every Friday.
+  # every 6 weeks, on a Thursday; this event runs every Friday. We use this
+  # moment to update CI caches.
   schedule:
     - cron: '0 10 * * FRI'
 
+  # Allow manual execution of CI, primarily for debugging. We consider these
+  # rare and also update CI caches when they occur.
+  workflow_dispatch:
+
 defaults:
   run:
     shell: bash
@@ -300,7 +305,7 @@ jobs:
 
     # Restore a cached `target/`.
     - name: Restore from cache
-      if: github.event_name != 'schedule'
+      if: github.event_name != 'schedule' && github.event_name != 'workflow_dispatch'
       id: cache-restore
       uses: actions/cache/restore@v6
       with:
@@ -329,7 +334,7 @@ jobs:
 
     # Note the Rust version in the cache.
     - name: Note Rust version for the cache
-      if: github.event_name == 'schedule'
+      if: github.event_name == 'schedule' || github.event_name == 'workflow_dispatch'
       run: |
         # NOTE: `rustc --version --verbose` also prints a `commit-date`, which
         # is easier to parse, but it is unclear whether it perfectly matches
@@ -347,7 +352,7 @@ jobs:
     # Save to the cache every week.
     - name: Save to the cache
       uses: actions/cache/save@v6
-      if: github.event_name == 'schedule'
+      if: github.event_name == 'schedule' || github.event_name == 'workflow_dispatch'
       with:
         path: |
           ~/.cargo
@@ -418,7 +423,7 @@ jobs:
 
     # Save to the cache.
     - name: Save to the cache
-      if: github.event_name == 'schedule'
+      if: github.event_name == 'schedule' || github.event_name == 'workflow_dispatch'
       uses: actions/cache/save@v6
       with:
         path: |