ci: Allow running mingw tests by default via environment variable

Hi,

We have CI support for mingw, but don't run the task by default, as it eats up
precious CI credits. However, for cfbot we are using custom compute resources
(currently donated by google), so we can afford to run the mingw tests. Right
now that'd require cfbot to patch .cirrus.tasks.yml.

While one can manually trigger manual task in one one's own repo, most won't
have the permissions to do so for cfbot.

I propose that we instead run the task automatically if
$REPO_MINGW_TRIGGER_BY_DEFAULT is set, typically in cirrus' per-repository
configuration.

Unfortunately that's somewhat awkward to do in the cirrus-ci yaml
configuration, the set of conditional expressions supported is very
simplistic.

To deal with that, I extended .cirrus.star to compute the required environment
variable. If $REPO_MINGW_TRIGGER_BY_DEFAULT is set, CI_MINGW_TRIGGER_TYPE is
set to 'automatic', if not it's 'manual'.

We've also talked in other threads about adding CI support for
1) windows, building with visual studio
2) linux, with musl libc
3) free/netbsd

That becomes more enticing, if we can enable them by default on cfbot but not
elsewhere. With this change, it'd be easy to add further variables to control
such future tasks.

I also attached a second commit, that makes the "code" dealing with ci-os-only
in .cirrus.tasks.yml simpler. While I think it does nicely simplify
.cirrus.tasks.yml, overall it adds lines, possibly making this not worth it.
I'm somewhat on the fence.

Thoughts?

On the code level, I thought if it'd be good to have a common prefix for all
the automatically set variables. Right now that's CI_, but I'm not at all
wedded to that.

Greetings,

Andres Freund