This way mingw-w64 becomes the first CI workflow to build gpu-next.
Unfortunately, currently neither wscript or meson has gpu-next or
libplacebo-next as an option that one can require.
Update the github workflows to also do meson builds for every OS.
Additionally, make every workflow execute the built mpv executable
(except for windows and FreeBSD's waf executable) to make sure that it
runs. As an aside, FreeBSD unfortunately is a bit less elegant since it
is in a VM.
Python 2 may not be present in the CI images in the future,
but waf’s shebang line still uses its executable name.
Explicitly call the right major version of the interpreter.
