ci(spm): set Hafnium default LLVM path

From change Iefb14564554f7d907751ba82db6884fe7fa5c50c Hafnium requires
LLVM toolchain path to be specified in the PATH environment variable.
Set it to the toolchain stored in the project prebuilts submodule for
convenience in the CI.

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I5ad4f9f31cf151ddb227ae6d856d4095fdd03c95
1 file changed