They use powershell instead of cmd.exe, use the tag win32-ps instead of win32,
and run Windows 2016 instead of 2012r2.
The old runners will be switched off in the comming weeks.
A recent dependency change in MSYS2 made it pull in vulkan, which made
meson think it's available but it somehow links against the system vulkan dll
instead.
Disable vulkan for now.
There where some problems (??) with ccache not detecting changes during meson
checks. Setting CCACHE_DISABLE during the meson execution makes ccache not use
the cache and pass things directly to the compiler.
