ci: Per-repo configuration for manually trigger tasks

We do not want to trigger some tasks by default, to avoid using too many
compute credits. These tasks have to be manually triggered to be run. But
e.g. for cfbot we do have sufficient resources, so we always want to start
those tasks.

With this commit, an individual repository can be configured to trigger
them automatically using an environment variable defined under
"Repository Settings", for example:

REPO_CI_AUTOMATIC_TRIGGER_TASKS="mingw netbsd openbsd"

This will enable cfbot to turn them on by default when running tests for the
Commitfest app.

Backpatch this back to PG 15, even though PG 15 does not have any manually
triggered task. Keeping the CI infrastructure the same seems advantageous.

Author: Andres Freund <andres@anarazel.de>
Co-authored-by: Thomas Munro <thomas.munro@gmail.com>
Co-authored-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/20240413021221.hg53rvqlvldqh57i%40awork3.anarazel.de
Backpatch-through: 16

src/tools/ci/README

@@ -82,3 +82,14 @@ defined in .cirrus.yml, by redefining the relevant yaml anchors.
 Custom compute resources can be provided using
 - https://cirrus-ci.org/guide/supported-computing-services/
 - https://cirrus-ci.org/guide/persistent-workers/
+
+
+Enabling manual tasks by default
+================================
+
+Some tasks are not triggered automatically by default, to avoid using up CI
+credits too quickly. This can be changed on the repository level, e.g. when
+custom compute resources are configured.
+
+The following repository level environment variables are recognized:
+- REPO_CI_AUTOMATIC_TRIGGER_TASKS=""