selftest/virgl: use pkg-config from the host

The check needs to report dri location on the host machine,
so pkg-config binary needs to be capable of finding the needed
dri.pc file on the host, and therefore needs to know where
host .pc files are located.

This may not be the case when using pkg-config from buildtools,
so this forces usage of host pkg-config.

runqemu already does the same PATH tweak, so this simply brings
the two in sync.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>