workflows/bot: allow maintainer merges after committer approval

This allows committers to approve PRs with additional, optional nits
that the author-maintainer can either address or merge immediately
without these changes.

It also allows committers to approve a PR for merge, while still waiting
for other maintainers to give their feedback - they can then merge the
PR directly instead of passing it back to the committer.

ci/github-script/merge.js

@@ -2,6 +2,7 @@ const { classify } = require('../supportedBranches.js')
 
 function runChecklist({
   committers,
+  events,
   files,
   pull_request,
   log,
@@ -23,11 +24,25 @@ function runChecklist({
         .map((pkg) => new Set(maintainers[pkg]))
         .reduce((acc, cur) => acc?.intersection(cur) ?? cur)
 
+  const approvals = new Set(
+    events
+      .filter(
+        ({ event, state, commit_id }) =>
+          event === 'reviewed' &&
+          state === 'approved' &&
+          // Only approvals for the current head SHA count, otherwise authors could push
+          // bad code between the approval and the merge.
+          commit_id === pull_request.head.sha,
+      )
+      .map(({ user }) => user.id),
+  )
+
   const checklist = {
     'PR targets a [development branch](https://github.com/NixOS/nixpkgs/blob/-/ci/README.md#branch-classification).':
       classify(pull_request.base.ref).type.includes('development'),
     'PR touches only packages in `pkgs/by-name/`.': allByName,
     'PR is at least one of:': {
+      'Approved by a committer.': committers.intersection(approvals).size > 0,
       'Authored by a committer.': committers.has(pull_request.user.id),
       'Backported via label.':
         pull_request.user.login === 'nixpkgs-ci[bot]' &&
@@ -246,6 +261,7 @@ async function handleMerge({
 
     const { result, checklist } = runChecklist({
       committers,
+      events,
       files,
       pull_request,
       log,
@@ -302,6 +318,7 @@ async function handleMerge({
 
   const { result } = runChecklist({
     committers,
+    events,
     files,
     pull_request,
     log,