ci: add workflow_dispatch to scheduled workflows (#16862)

This will allow anyone with write access to [trigger these workflows manually](https://docs.github.com/en/actions/managing-workflow-runs-and-deployments/managing-workflow-runs/manually-running-a-workflow) without waiting for the scheduled time. Useful for re-running if something was broken and for testing.

I also added / updated comments giving the cron schedules in words (using https://crontab.guru).

[Zulip](https://leanprover.zulipchat.com/#narrow/stream/287929-mathlib4/topic/Technical.20Debt.20Counters/near/470626800)

.github/workflows/dependent-issues.yml

@@ -3,6 +3,7 @@ name: Dependent Issues
 on:
   schedule:
     - cron: '*/15 * * * *' # run every 15 minutes
+  workflow_dispatch:
 
 jobs:
   cancel:

.github/workflows/lean4checker.yml

@@ -4,6 +4,7 @@ name: lean4checker Workflow
 on:
   schedule:
     - cron: '0 0 * * *'   # Runs at 00:00 UTC every day
+  workflow_dispatch:
 
 jobs:
   check-lean4checker:

.github/workflows/merge_conflicts.yml

@@ -3,6 +3,7 @@ name: Merge conflicts
 on:
   schedule:
     - cron: '*/15 * * * *' # run every 15 minutes
+  workflow_dispatch:
 
 jobs:
   main:

.github/workflows/nightly_bump_toolchain.yml

@@ -1,9 +1,9 @@
 name: Bump lean-toolchain on nightly-testing
 
 on:
-  workflow_dispatch:
   schedule:
     - cron: '0 10/3 * * *'  # Run every three hours, starting at 11AM CET/2AM PT.
+  workflow_dispatch:
 
 jobs:
   update-toolchain:

.github/workflows/nightly_merge_master.yml

@@ -4,7 +4,8 @@ name: Merge master to nightly
 
 on:
   schedule:
-    - cron: '30 */3 * * *'  # 8AM CET/11PM PT
+    - cron: '30 */3 * * *'  # At minute 30 past every 3rd hour.
+  workflow_dispatch:
 
 jobs:
   merge-to-nightly:

.github/workflows/nolints.yml

@@ -2,7 +2,8 @@ name: update nolints
 
 on:
   schedule:
-    - cron: "0 0 * * 0"
+    - cron: "0 0 * * 0" # At 00:00 UTC on Sunday.
+  workflow_dispatch:
 
 jobs:
   build:

.github/workflows/technical_debt_metrics.yml

@@ -2,7 +2,8 @@ name: Weekly Technical Debt Counters
 
 on:
   schedule:
-    - cron: '0 4 * * 1'  # Run at 04:00 every Monday
+    - cron: '0 4 * * 1'  # Run at 04:00 UTC every Monday
+  workflow_dispatch:
 
 jobs:
   run-script:

.github/workflows/update_dependencies.yml

@@ -3,6 +3,7 @@ name: Update Mathlib Dependencies
 on:
   schedule:
     - cron: '0 * * * *'  # This will run every hour
+  workflow_dispatch:
 
 jobs:
   update-dependencies: