[CI] Disable liboffload job

[RossBrunton]

The packages it requires haven't been updated since the 3rd of July (as
of this commit), so disable for now.

[sarnex]

LGTM for now but could we try building LLVM as part of the CI job, or having a prebuilt one available somewhere that's updated at the needed interval?

[RossBrunton]

@intel/llvm-gatekeepers Please merge.

EDIT: Whoops, there's still a check waiting.

@sarnex Ignoring any caching, that would involve an extra build of LLVM for each offload job, which might be a bit much.

[sarnex]

We could create a docker container that has it prebuilt and only update that manually as necessary or something like that. Either way, up to you.