Re: Mingw task for Cirrus CI

It's a bit odd to separate the CCACHE_* variables from each other
(e.g. BUILD_DIR is inbetween them)...

Perhaps add a comment explaining that otherwise plpython tests fail? 

With this src/tools/ci/cores_backtrace.sh shouldn't need to be modified
anymore afaict?

I'd replace --buildtype debug with -Ddebug=true -Doptimization=g.

Seems like this could use %MTEST_ARGS%?

> +    CCACHE_SLOPPINESS: pch_defines,time_macros
> +    CCACHE_DEPEND: 1

I experimented a bit and it looks like ccache doesn't yet quite work in CI,
but only because the ccache needs to be larger. Looks like we need about
~400MB.

A fully cached build is ~2min