shm: Pick the shm dir at run time, not build time.

Prodding the builder's filesystem for tmp dirs doesn't necessarily
tell you anything about what the actual host's filesystem is going to
look like, so we should just try the dirs at runtime.

Signed-off-by: Eric Anholt <eric@anholt.net>
1 job for runtime-shmdir in 1 minute and 28 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Test
passed job #24242

00:01:28