fix msys ci python issue

[lkeegan]

Description

• msys ci: un-pin setup-msys2 action version
• msys ci: explicitly set PYTHONHOME and PYTHONPATH for c++ and interface tests (to workaround Python broke in v2.5.0 msys2/setup-msys2#167)
• see WIP Fixing MINGW CI failures #3646

Suggested changelog entry:

[Skylion007]

Is there really not a more elegant way to fix this? ;-;

[rwgk]

Thanks a ton Liam!!