[plugin-host] change THEIA_ env exclusion filter

[AlexTugarev]

to match ^THEIA_ instead of THEIA_. this allows to access PREFIX_THEIA_SUFFIX env vars in plug-ins.

Fixes #7084

How to test

• export PREFIX_THEIA_FOO env var before launching theia backend process
• use e.g. python vscode extension to debug some code and execute import os; print(os.environ['PREFIX_THEIA_FOO']) to validate

Review checklist

• as an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• as a reviewer, I agree to behave in accordance with the review guidelines

[svenefftinge]

LGTM

[AlexTugarev]

Thanks @svenefftinge and @benoitf!
I'm merging despite of test failures on windows as they looks unrelated.