Add --gemfile flag to bundle pristine

[martinemde]

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

I just switched computers and my gems needed to be rebuilt. I first tried bundle pristine --gemfile tool/bundler/vendor_gems.rb and it didn't work.

I ended up with gem pristine --all but it still seemed odd that the --gemfile flag didn't exist for pristine.

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

Add --gemfile to bundle pristine.

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

[deivid-rodriguez]

Needs some test tweaks according to CI but yeah, I don't see why this command should not take a --gemfile flag!

[martinemde]

Planning to fix the tests soon. 😵‍💫