CI: Correct build-directory path on Windows, and keep it

CI: Correct build-directory path on Windows, and keep it

Build job artifacts capture Meson logs from _build, so we can analyse
what Meson did during configuration, as well as the full output of any
test jobs.

We were previously calling our build directory 'build', which meant it
wouldn't have been captured by the artifacts, and we were also deleting
it to make really sure there was no chance of logs getting captured
either.

Rename the build directory to '_build' to match the others, and don't
delete it either, so we can keep our configure/test logs.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5689>