Set the Bundler UI level to silent to prevent output to stdout

[vinistock]

Motivation

Fixes #1723

If users have Bundler's logging level configured to info or debug, internals will print to stdout and mess up the client/server communication. This happens when invoking methods like Bundler.locked_gems or specs. Here's an example.

We use these methods in two places

• For setting up the custom bundle (which should hopefully go away as soon as bundler-compose is stable for usage
• For figuring out which gems to index (won't go away)

Implementation

Set the Bundler UI level to silent so that it does not print anything regardless of user configurations. One of them will go away once we move to bundler-compose and just the one in internal will remain.

[andyw8]

Note: This might not be working correctly if BUNDLE_AUTO_INSTALL is enabled.