ci: rework vulkan validation layer build script

Initially this was just adding a missing popd, but actually there's no reason to pushd into the build dir, so let's just pass the build dir as arguments to cmake & ninja instead.

--arch x64 was also dropped as it only applies to Windows builds, which this script doesn't support anyway.

Fixes: 512f1c16 ("ci/zink: Add coverage using the vulkan validation layer on lvp.")