buildEnv: Check the content of colliding paths.

Originally wanted to include `ignoreCollisions` in cups-progs, but I think it's better if we use `ignoreCollisions` only if there are _real_ collisions between files with different contents.

Of course, we also check whether the file permissions match, so you get a collision if contents are the same but the permissions are different.

Cc for review to @edolstra and @iElectric