ci: enable custom env. vars in pkg check #899

Job	Run time
check-docs / make-docs (html)	2m 38s
check-md-links-default / markdown-link-check	21s
check-docs / make-docs (doctest)	2m 45s
check-md-links-w-config / markdown-link-check	45s
check-docs / make-docs (linkcheck)	1m 35s
check-package-extras / init-store	3s
check-package / init-store	3s
check-schema-fixed / schema	13s
check-precommit / pre-commit	10s
check-schema-latest / schema	13s
check-typing / mypy	16s
check-package / pkg-build (ubuntu-22.04, 3.10)	15s
check-package-extras / pkg-build (ubuntu-latest)	15s
check-schema	1s
check-package / pkg-check (ubuntu-22.04, 3.8)	19s
check-package / pkg-check (ubuntu-22.04, 3.12)	13s
check-package / pkg-check (macos-12, 3.8)	32s
check-package / pkg-check (macos-12, 3.12)	33s
check-package / pkg-check (windows-2022, 3.8)	47s
check-package / pkg-check (windows-2022, 3.12)	55s
check-package-extras / pkg-check (ubuntu-22.04, 3.10)	28s
check-package-extras / pkg-check (macos-12, 3.10)	40s
check-package-extras / pkg-check (windows-2022, 3.10)	49s
check-package-extras / pkg-guardian	1s
check-package / pkg-guardian	1s
	14m 51s

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