piglit: replace setting PIGLIT_BUILD_DIR with a correct reproducibility fix

piglit: replace setting PIGLIT_BUILD_DIR with a correct reproducibility fix

PIGLIT_BUILD_DIR was set to source tree location relative to B as a
reproducibility workaround some years ago. Aafter some investigation
I found where the actual issue is (addressed by the patch).

Also, setting PIGLIT_BUILD_DIR relative to ${B} in a hardcoded way
is problematic as S and B are not necessarily related that way (e.g. when
sources are in UNPACKDIR or somewhere else entirely).

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>