cmd/shellenv: fix for tcsh #38226

Job	Run time
syntax	1m 15s
cask audit	2m 20s
update-test (Linux)	26s
tap syntax	5m 56s
vendored gems	54s
update-test (macOS)	41s
formula audit	7m 9s
tests (Linux)	9m 55s
bundle and services (macOS)	4m 7s
tests (online)	15m 3s
tests (macOS)	14m 26s
bundle and services (Linux)	6m 9s
analytics (macOS)	2m 7s
test-bot (Linux x86_64)	1m 55s
analytics (Linux)	2m 7s
test-bot (Linux Homebrew glibc)	2m 21s
test-bot (macOS x86_64)	5m 35s
test-bot (macOS arm64)	1m 42s
tests (generic OS)	7m 40s
test-bot (Linux arm64)	2m 18s
	1h 34m 6s

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