GHA: Test MSYS2/CLANG64

[nulano]

MSYS2 now has more environments: https://www.msys2.org/docs/environments/

Changes proposed in this pull request:

• Test CLANG64 instead of MINGW32, GHA: Test MSYS2/CLANG64 #6888 (comment)
• Add xfail mark for Flaky test: test_qt_image_qapplication.py::test_sanity #6875 as it failed 9 times in a row
• Update installation documentation
• While investigating Enable gcov support for codecov/codecov-action #6944, I found that replacing the commands used in the workflow with the .ci scripts simplifies it a bit.

[radarhere]

I've rerun the tests, and they've passed now.

[hugovk]

Thank you for this PR!

What's the benefit in adding all of these environments?

Our build matrix is pretty big right now, so let's be careful not to make it too much bigger.

This increases the test jobs in this worfklow from 2 to 5, and total duration from ~12 to ~21 mins:

That's on top of 61 checks at the moment (mostly but not all GHA), taking about 184 mins of GHA time (wall-clock time maybe about an hour?).

For example, we aren't testing the GHA macOS/Ubuntu jobs with both architectures, and we will need to add some 3.12-dev coverage soonish.

Not opposed to this PR, and I'm very much for increasing test coverage, but let's keep an eye on things so they stay manageable :)

[kmilos]

IMHO testing MINGW64 (GCC toolchain w/ legacy MSVCRT) and CLANG64 (LLVM toolchain w/ modern UCRT) should provide decent coverage if one is trying to reduce the number of jobs. There's little point in testing 32-bit, support for it is being phased out anyway...

[nulano]

I've had a hardware issue preventing me from working on this until now...

What's the benefit in adding all of these environments?

I was going to suggest something similar to @kmilos's comment.

I've now updated the test matrix to (and listed the others in a comment):

• CLANG64 instead of MINGW32
• MINGW64

[kmilos]

Any particular reason for not using the setup-msys2 action and all this extra manual work?

[nulano]

IIRC this was made before the setup-msys2 action, and it seems like less work to just update it as is instead of switching to the setup-msys2 action.

[kmilos]

I think you'll find switching will pay off when it comes to flexibility and long term maintanance of these jobs.

I might be able to help with this when I find some time.