ci/windows: work around meson encoding issues

It turns out, some combinations of Meson and MSVC doesn't handle UTF-8
symbols in source-files as gracefully as we'd like. Luckily, there's a
work-around; forcing UTF-8 as the default-encoding.

Please see this Meson ticket for details:
https://github.com/mesonbuild/meson/issues/8263

Cc: mesa-stable
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Acked-by: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11424>

.gitlab-ci/windows/mesa_build.ps1

@@ -2,6 +2,8 @@
 Write-Host "Refreshing Windows TLS CA cache"
 (New-Object System.Net.WebClient).DownloadString("https://github.com") >$null
 
+$env:PYTHONUTF8=1
+
 Get-Date
 Write-Host "Compiling Mesa"
 $builddir = New-Item -ItemType Directory -Name "_build"