Build DMD, Druntime + Phobos in debug mode

[wilzbach]

Not sure whether it's a good idea as this might result in a slowdown.
Also in theory there might be differences between the debug and release version and thus could result in an undetected breakage.

OTOH seeing a nice stacktrace on errors at the CI helps, though of course it can be argued that this is needed so seldom that it's not worth the risk or slowdown.

[dlang-bot]

Thanks for your pull request, @wilzbach!

[MartinNowak]

Somewhat undecided.

This goes back to building dmd with assertions but without the invariant slowdown.
dlang/dmd#7980
Though the test suite doesn't spend so much time building code I guess.
Another issue is the up&download size of the binary artifacts, hopefully compressions works well enough here.