chore(ci): send mail when daily integration tests fail (#8727)

## Testing

- 24h after this is merged
- push a commit to https://codeberg.org/forgejo-integration/forgejo/src/branch/forgejo with an error
- cancel all workflows except for https://codeberg.org/forgejo-integration/forgejo/actions?workflow=testing-integration.yml&actor=0&status=0
- verify a notification was sent to forgejo-integration-actions@forgejo.org about the error
- update the user research discussion at https://codeberg.org/forgejo/user-research/issues/64

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8727
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Co-authored-by: Earl Warren <contact@earl-warren.org>
Co-committed-by: Earl Warren <contact@earl-warren.org>

.forgejo/workflows/testing-integration.yml

@@ -1,7 +1,8 @@
 #
 # Additional integration tests designed to run once a day when
 # `mirror.yml` pushes to https://codeberg.org/forgejo-integration/forgejo
-# and send a notification via email should they fail.
+# and send a notification via email to the contact email of the
+# organization should they fail.
 #
 # For debug purposes:
 #
@@ -22,6 +23,8 @@ on:
       - 'forgejo'
       - 'v*/forgejo'
 
+enable-email-notifications: true
+
 jobs:
   test-unit:
 #    if: vars.ROLE == 'forgejo-coding'