Creating a temporary config directory for the in-build GIMP (run as a tool or
for unit-testing) is not done as a build target anymore, but in the
in-build-gimp.sh script as a unique temp directory, then cleaned out on exit.
This has a few advantages:
- It is properly cleaned out once the build ends (instead of leaving a full
config dir as trash inside the build dir).
- It is not reused from one build to another (with risk of carrying bugs and
issues over).
- Every use of the in-build GIMP will have its own config directory, and in
particular when they are called in parallel.
As a side update, make sure that all `gimp_exe` runs depend on
`gimp_exe_depends`.
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