fix: keep RUBYLIB env var

[jdx]

This should not be necessary anymore now that ruby is a core plugin.

Fixes #2380

[github-actions]

Package	Line Rate	Complexity	Health
src.backend	69%	0	➖
src	86%	0	✔
src.cli	83%	0	✔
src.cli.alias	89%	0	✔
src.cli.args	97%	0	✔
src.cli.backends	95%	0	✔
src.cli.cache	95%	0	✔
src.cli.config	92%	0	✔
src.cli.direnv	80%	0	✔
src.cli.generate	87%	0	✔
src.cli.plugins	86%	0	✔
src.cli.settings	93%	0	✔
src.cli.sync	27%	0	❌
src.cli.tasks	71%	0	➖
src.config.config_file	89%	0	✔
src.config	87%	0	✔
src.plugins	90%	0	✔
src.plugins.core	56%	0	➖
src.shell	98%	0	✔
src.toolset	86%	0	✔
src.ui	63%	0	➖
Summary	80% (13685 / 17038)	0	✔

[codacy-production]

Coverage summary from Codacy

See diff coverage on Codacy

Coverage variation	Diff coverage
✅ +0.00%	✅ ∅

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (a897efa)	17039	13686	80.32%
Head commit (52158d8)	17038 (-1)	13685 (-1)	80.32% (+0.00%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#2387)	0	0	∅ (not applicable)

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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