Avoid propagating link_arg values that are unlikely to be resolveable under arbitrary directories.
auto merge of #5332 : jdm/rust/transitivelink, r=graydon

The original change bit Servo because rust-harfbuzz includes libharfbuzz.a in its link_args. This works fine in the rust-harfbuzz subdirectory where the static library resides, but when this is propagated to servo_gfx, the lirbrary can no longer be found since it's a relative path.