fix(test): add #[serial] to env-var tests to prevent concurrent acces… #7093

Job	Run time
optimize-ci	5s
detect-changes	6s
Test (Linux x64 musl)	2m 55s
Lint	1m 1s
Test (namespace-profile-linux-x64-default, x86_64-unknown-linux-gnu)	1m 33s
download-previous-rolldown-binaries	10s
Test (namespace-profile-mac-default, aarch64-apple-darwin)	1m 19s
Test (windows-latest, x86_64-pc-windows-msvc)	4m 8s
CLI E2E test (Linux x64 musl)	5m 44s
CLI E2E test (windows-latest, x86_64-pc-windows-msvc)	9m 49s
CLI E2E test (namespace-profile-linux-x64-default, x86_64-unknown-linux-gnu)	3m 36s
Local CLI `vp install` E2E test	7m 10s
CLI E2E test (namespace-profile-mac-default, aarch64-apple-darwin)	3m 16s
CLI snap test (aarch64-apple-darwin, 2/3)	4m 18s
CLI snap test (x86_64-unknown-linux-gnu, 3/3)	5m 32s
CLI snap test (x86_64-unknown-linux-gnu, 2/3)	5m 39s
CLI snap test (x86_64-unknown-linux-gnu, 1/3)	5m 29s
CLI snap test (x86_64-pc-windows-msvc, 3/3)	10m 2s
CLI snap test (x86_64-pc-windows-msvc, 1/3)	9m 57s
CLI snap test (x86_64-pc-windows-msvc, 2/3)	10m 11s
CLI snap test (aarch64-apple-darwin, 1/3)	4m 0s
CLI snap test (aarch64-apple-darwin, 3/3)	4m 20s
done	3s
	1h 40m 23s

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