Investigate disabling `BUNDLE_BIN` automatically

[vinistock]

Apparently, there's a BUNDLE_BIN option to automatically create binstubs for installed gems. I was not aware of this setting, but we need to turn it off when setting up the custom bundle or else we accidentally create binstubs for the application.

[andyw8]

Let's review the others too: https://bundler.io/v2.5/man/bundle-config.1.html#LIST-OF-AVAILABLE-KEYS

(perhaps it may be safe to filter on a set that we do allow).

[vinistock]

Yeah, that's a good idea.