cmd/shellenv: set `XDG_DATA_DIRS` on Linux #30199

Job	Run time
syntax	52s
tap syntax	5m 2s
cask audit	3m 2s
formula audit	7m 18s
update-test (Ubuntu 22.04)	21s
update-test (macOS)	26s
vendored gems	40s
tests (online)	9m 51s
test default formula (Ubuntu 22.04)	4m 22s
tests (generic OS)	5m 56s
test default formula (Ubuntu 20.04)	6m 39s
tests (Ubuntu 22.04)	6m 2s
test default formula (macOS 13 x86_64)	2m 32s
tests (Ubuntu 20.04)	7m 55s
test default formula (macOS 14 arm64)	1m 30s
tests (macOS 13 x86_64)	7m 55s
tests (macOS 14 arm64)	11m 56s
	1h 22m 19s

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