fix(ci): respect retain_paths when downloading llvm

Most workstations have LLVM installed. Most developers will have
retain_paths set which is meant to skip some downloads. Respect this for
the llvm download so that CI can run more quickly locally.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I14c8de37613067bc0a9187411504d8ccd873bd3b
1 file changed