CI: windows: Force using LLVM 12

CI: windows: Force using LLVM 12

LLVM is moving to the 13 release, but LLVM-SPIRV is still so in the past.
Given that LLVM 12.0.0 is still not out (we are at 12.0.0-rc1 today),
use the `release/12.x` branch for LLVM.

We should also tag LLVM-SPIRV, but... it seems that they haven't caught up
yet, so keep using the master branch, but add a note for a future
committer.

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Daniel Stone <daniel@fooishbar.org>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8740>