Specify force_ci/** branch pattern that can force a workflow to run

The previous commit restricting build.yml to only run on 'main'
(same as newly-introduced debian.yml). But it's useful to be able
to see CI runs, during development _before_ opening a pull request.

A branch pattern is added so that forcing CI to run is possible.

.github/workflows/build.yml

@@ -5,7 +5,9 @@ on:
   pull_request:
   push:
     branches:
-      - main
+      - 'main'
+      - 'force_ci/all/**'    # For development, forcing all workflows to run.
+      - 'force_ci/build/**'  # For debugging and/or only forcing this workflow.
 
 jobs:
   # Build the RocksDB C library and cache the result.

.github/workflows/debian.yml

@@ -4,8 +4,9 @@ on:
   pull_request:
   push:
     branches:
-      - main
-      - workflows/debian  # For debugging.
+      - 'main'
+      - 'force_ci/all/**'     # For development, forcing all workflows to run.
+      - 'force_ci/debian/**'  # For debugging and/or only forcing this workflow.
 
 jobs:
   debian-build: