Adjust tox workflow for improved integration

- Remove `fail-fast: false` to allow failing checks to cancel
  others. This is to avoid unnecessary usage and wait times. It may
  be necessary to undo this temporarily again in the future when
  investigating failures, for failures that happen due to external
  changes rather than to a `subaudit` code change.

- Add a "collector" job that depends on all the jobs generated by
  the `tox` matrix job definition, so the individual jobs don't
  have to be added as required checks for branch protection.
  Instead, the name "All tox checks pass" has been added.

  This is based indirectly on the `test-pass` job in `gitoxide`,
  which was introduced in GitoxideLabs/gitoxide#1551 by
  https://github.com/NobodyXu. It is more specifically based on my
  adaptation of that in `algorithms-python` introduced in
  EliahKagan/algorithms-python#234.

  This is a common pattern, so it does not necessarily resemble
  the code in `gitoxide` more than it does that pattern elsewhere,
  and it is simpler than the version in `algorithms-python`. See:
  EliahKagan/algorithms-python@60d2d4e

Author: EliahKagan

.github/workflows/tox.yml

@@ -112,8 +112,6 @@ jobs:
         include:
           - experimental: false
 
-      fail-fast: false
-
     continue-on-error: ${{ matrix.experimental }}
 
     runs-on: ${{ matrix.os }}
@@ -139,3 +137,21 @@ jobs:
 
       - name: Check with tox
         run: tox
+
+  all-pass:
+    name: All tox checks pass
+
+    needs:
+      - tox
+
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Some failed
+        if: contains(needs.*.result, 'cancelled') || contains(needs.*.result, 'failure')
+        run: |
+          false
+
+      - name: All passed
+        run: |
+          true