runqueue/bitbake-worker: Fix dry run fakeroot issues

When using the dry run option (-n), bitbake would still try and fire
a specific fakeroot worker. This is doomed to failure since it might
well not have been built.

Add in some checks to prevent the failures.

[YOCTO #5367]

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>