CI: Checkout driver tag into the directory we build from

It was previously checking out the default branch into the $DRIVER subdirectory, and the requested tag into the current subdirectory, but then proceeding to build the $DRIVER subdirectory.

Fixes: 89b7f450 ("CI: add a driver build stage to check for header breakage") from !1318 (merged)

Noticed because my other MR today had a failed CI job https://gitlab.freedesktop.org/xorg/xserver/-/jobs/61020243 in which configure failed for xf86-video-amdgpu because Requested 'libdrm >= 2.4.121' but version of libdrm is 2.4.116 - but the change to use 2.4.121 just merged this week (xorg/driver/xf86-video-amdgpu@04bc88d9) and isn't included in the listed tag for the amdgpu driver.