labels: add workflow related labels

This adds some labels related to the higher level workflows of
developing nixpkgs:
- "4.workflow: backport" can be used to filter everything targeting the
stable branches, both release-XX.YY or staging-XX.YY.
- "4.workflow: package set updates" can be used to filter the
haskell/python package set PRs. Can be combined with "6.topic: haskell"
or "6.topic: python".
- "4.workflow: staging" can be used to filter for all staging-next PRs,
both to master and release branches.

.github/labeler-protected-branches.yml

@@ -1,6 +1,17 @@
 # This file is used by .github/workflows/labels.yml
 # This version is only run for Pull Requests from protected branches like staging-next, haskell-updates or python-updates.
 
+"4.workflow: package set update":
+  - any:
+    - head-branch:
+      - '-updates$'
+
+"4.workflow: staging":
+  - any:
+    - head-branch:
+      - '^staging-next$'
+      - '^staging-next-'
+
 "6.topic: haskell":
   - any:
     - head-branch:

.github/labeler.yml

@@ -3,6 +3,12 @@
 
 # keep-sorted start case=no numeric=yes newline_separated=yes skip_lines=1
 
+"4.workflow: backport":
+  - any:
+    - base-branch:
+      - '^release-'
+      - '^staging-'
+
 # NOTE: bsd, darwin and cross-compilation labels are handled by ofborg
 "6.topic: agda":
   - any: