Clone SPIRV-Tools and Headers in S instead of WORKDIR so they don't
escape file-prefix-map substitutions.
Fixes buildpaths warnings:
| WARNING: intel-graphics-compiler-1.0.12812.9-r0 do_package_qa: QA Issue: File /usr/lib/.debug/libigc.so.1.0.1 in package intel-graphics-compiler-dbg contains reference to TMPDIR [buildpaths]
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>