In order to be able to change GIMP2_DIRECTORY during run-time, check
for changes to GIMP2_DIRECTORY in gimp_directory().

This is typically useful in test case where you could read from one
GIMP2_DIRECTORY at start-up and then write to a different
GIMP2_DIRECTORY at shut down.

The documentation for this function does not suggest that the value is
cached and thus the old behavior can be considered a bug. It is hard
to imagine why anyone would change GIMP2_DIRECTORY at run-time in a
script for example and *not* expect gimp_directory() to return the
updated gimp_dir.