Introduce a gate/check GHA job

This adds a GHA job that reliably determines if all the required
dependencies have succeeded or not.

It also allows to reduce the list of required branch protection CI
statuses to just one — `check`. This reduces the maintenance burden
by a lot and have been battle-tested across a small bunch of projects
in its action form and in-house implementations of other people.

This action is now in use in aiohttp (and other aio-libs projects),
CherryPy, conda, coveragepy, Open edX, Towncrier some of the Ansible
repositories, pip-tools, all of the jaraco's projects (like
`setuptools`, `importlib_metadata`), some of hynek's projects (like
`attrs`, `structlog`), some PyCQA, PyCA, PyPA and pytest projects, a
few AWS Labs projects. Admittedly, I maintain a few of these but it
seems to address some of the pain folks have:
jaraco/skeleton#55 (comment).

I figured, this might be useful for CPython too, which is why I'm
submitting this patch.

The story behind this is explained in more detail at
https://github.com/marketplace/actions/alls-green#why.

.github/workflows/build.yml

@@ -117,6 +117,7 @@ jobs:
     runs-on: windows-latest
     needs: check_source
     if: needs.check_source.outputs.run_tests == 'true'
+    continue-on-error: true
     env:
        IncludeUwp: 'true'
     steps:
@@ -153,6 +154,7 @@ jobs:
     runs-on: macos-latest
     needs: check_source
     if: needs.check_source.outputs.run_tests == 'true'
+    continue-on-error: true
     env:
       PYTHONSTRICTEXTENSIONBUILD: 1
     steps:
@@ -232,6 +234,7 @@ jobs:
     runs-on: ubuntu-20.04
     needs: check_source
     if: needs.check_source.outputs.run_tests == 'true'
+    continue-on-error: true
     strategy:
       fail-fast: false
       matrix:
@@ -318,3 +321,38 @@ jobs:
       run: make pythoninfo
     - name: Tests
       run: xvfb-run make buildbottest TESTOPTS="-j4 -uall,-cpu"
+
+  check:  # This job does nothing and is only used for the branch protection
+    if: always()
+
+    needs:
+    - check_source  # Transitive dependency, needed to access `run_tests` value
+    - check_generated_files
+    - build_win32
+    - build_win_amd64
+    - build_macos
+    - build_ubuntu
+    - build_ubuntu_ssltests
+    - build_asan
+
+    runs-on: ubuntu-latest
+
+    steps:
+    - name: Decide whether the needed jobs succeeded or failed
+      uses: re-actors/alls-green@13b4244b312e8a314951e03958a2f91519a6a3c9
+      with:
+        allowed-skips: >-
+          ${{
+            needs.check_source.outputs.run_tests == 'true'
+            && '
+            check_generated_files,
+            build_win32,
+            build_win_amd64,
+            build_macos,
+            build_ubuntu,
+            build_ubuntu_ssltests,
+            build_asan,
+            '
+            || ''
+          }}
+        jobs: ${{ toJSON(needs) }}