conf: add nice level to the hash config ignred variables

This is needed as each user could be setting different nice levels
while building, however this should not make the shared cache unusable.

Signed-off-by: Lorenzo Arena <arena.lor@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 42784f93)
Signed-off-by: Steve Sakoman <steve@sakoman.com>