Fix crash on read-only filesystems in Ruby 3.4

[Vasfed]

What was the end-user or developer problem that led to this PR?

This fixes #8369.

When running with readonly filesystem and no tmp directory on ruby 3.4 bundler raises exception on start

What is your fix for the problem, implemented in this PR?

While trying to check global installed plugins bundler internally tries to create tmp user home, which is not needed in this case. Added flag writable to Bundler.user_bundle_path(default true to keep previous behavior) to be able to determine whether intent is to have a writeable directory or nil will suffice.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
• Make sure you follow the current code style and write meaningful commit messages without tags

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[deivid-rodriguez]

Hi @Vasfed, thanks for the work on fixing this!

My first idea for the fix was something a bit different. It was doing it in two steps:

• For next patch level version, fix this by rescuing permission errors when finding the temporary directory in Bundler.user_home, and in that case, fallback to the real user home, even if it's unwritable. If Bundler ever needs to actually write to it, we'll get a permission error, which will be expected in readonly systems. Otherwise, things will just work, not only when checking plugins, but whenever this fallback actually happens.

• For next version allowing minor breaking changes, completely remove the fallback to a temporary user home. Always return the real home, and print a warning if it's not writable. In many years, I've never seen this fallback actually used, and I believe it causes more harm than good.

What do you think?

[Vasfed]

Hi @deivid-rodriguez at first only rescuing the exception was my first idea too, but in this case there're several warnings to stdout/stderr (from ruby itself and bundler).
Revisiting this - and indeed, I agree with you, for the patch version changes can be lesser - will change the PR accordingly.

About breaking changes - me too never encountered when tmp home is needed and there's no workaround. As for warning - may be print it only if an exception actually occurs

[Vasfed]

Changed to only rescue exception

[deivid-rodriguez]

Sorry for the delay @Vasfed. This looks great!

I rebased and made one change to convert the spec to a workflow that runs Bundler on a read-only system. The rationale being that the kind of test added in rubygems/bundler#6482 did not catch the issue introduced with Ruby 3.4, so I prefer to switch to something more realistic that will prevent the problem from being reintroduced in the future.

[deivid-rodriguez]

Thanks so much @Vasfed!