Fixes performance regression on pnpm installs

[goloveychuk]

What's the problem this PR addresses?

• We forgot the close the file descriptor when releasing JS files. This causes EMFILE on some jobs.
• The cache was mistakenly removed from the libzip backend, causing install times to jump, especially on pnpm installs.

Fixes #6758

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.

[arcanis]

I'd prefer shouldBeCached to be a class property than something returned by getFileSource, especially since it never changes (and probably doesn't have a good reason to ever change?).

[goloveychuk]

I was thinking that backend could decide whether to cache on per file basis.
E.g for big compressed files. Or frequently requested files. You decide.

[goloveychuk]

Please reply if you want per file or per class. I'll change tomorrow.

[arcanis]

Let's go with a class property. I think if we were to implement such a dynamic caching logic there's a good chance the checks you mention would live inside the ZipFS rather than the backend.

[goloveychuk]

Pushed