Use platformdirs to find user configuration files #699

Job	Run time
build-container-image	8s
Build dev-env (ubuntu-20.04)	1m 33s
Build dev-env (ubuntu-22.04)	1m 21s
Build dev-env (ubuntu-24.04)	1m 19s
Build dev-env (ubuntu-24.10)	1m 24s
Build dev-env (debian-bullseye)	1m 25s
Build dev-env (debian-bookworm)	1m 19s
Build dev-env (debian-trixie)	1m 27s
Build dev-env (fedora-40)	1m 34s
Build dev-env (fedora-41)	1m 23s
	12m 53s

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