The old "meson build" build command was actually deprecated a few months
ago*. It turns out that you're supposed to use "meson setup build"
instead which has been around for years. Go ahead and be a good citizen
and update this in the CI. Also replace any mention of "meson build"
with "meson setup build" in the documentation as well and change the one
random hardcoded string we have in meson.build to "meson configure
build" (might as well).
*: 3c7ab542c0
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.