Remove Dir.chdir for thread-safety

[lovro-bikic]

Currently, Bundler::Audit::Database#update!, #commit_id and #last_updated_at fail in multi-threaded environments (like background processing jobs) because they use Dir.chdir.

From Ruby docs on Dir.chdir:

In a multi-threaded program an error is raised if a thread attempts to open a chdir block while another thread has one open, or a call to chdir without a block occurs inside a block passed to chdir (even in the same thread).

This PR removes the need for Dir.chdir by using Open3.capture2 from the stdlib with the chdir option.

External behavior should stay the same: Dir.chdir raises an Errno::ENOENT error given an non-existent directory, and the same holds with the new implementation.

Fixes #156

[simi]

@lovro-bikic what about to use git -C (work dir), would that work also? 🤔

[lovro-bikic]

Nice one! I believe we could use it. I only have two questions:

1. -C option was introduced in v1.8.5. I'm not sure if we should support older Git versions?
2. While undocumented and untested, the gem currently raises Errno::ENOENT for unknown directories. With git -C, the error will look like below. Is that acceptable?

# with fake DB path "blabla"

> bin/bundler-audit update
Updating ruby-advisory-db ...
fatal: cannot change to 'blabla': No such file or directory