os/linux/diagnostic: remove XDG_DATA_DIRS. #31027

Job	Run time
syntax	39s
tap syntax	6m 46s
vendored gems	42s
formula audit	4m 36s
cask audit	2m 23s
test default formula (Ubuntu 22.04)	1m 34s
tests (online)	5m 0s
test default formula (Ubuntu 20.04)	3m 27s
tests (generic OS)	3m 2s
update-test (Ubuntu 22.04)	35s
test default formula (macOS 13 x86_64)	4m 44s
tests (Ubuntu 22.04)	3m 8s
update-test (macOS)	23s
test default formula (macOS 15 arm64)	1m 46s
tests (Ubuntu 20.04)	3m 47s
tests (macOS 13 x86_64)	9m 0s
tests (macOS 15 arm64)	4m 41s
	56m 13s

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