bundle install should honor BUNDLE_JOBS configuration

[mcartoixa]

We just ran into a bug related to Bundler where transitive dependencies would not resolve correctly when parallel installation (--jobs option) is enabled. The problem with the current action is that there is no way to configure this behaviour as it is hardcoded to 4 (cf. bundler.js).

It would be very helpful if the BUNDLE_JOBS configuration, if present, could be honored.

[eregon]

PR welcome. I don't recall why we pass --jobs 4, apparently since #56.
Probably it was experimented and found to be the fastest on the GH runners at the time.

[eregon]

Could link that Bundler issue here BTW?

[mcartoixa]

Sorry, I haven't reported it yet to Bundler. We maintain a set of legacy applications and we have observed it with versions 1.7 and 2.2 of Bundler because the last version of debase (0.2.7) does not compile in our environments. I still need to set up a minimal reproducible environment and check with the latest version of Bundler...
It is our experience that parallel installation is the default with Bundler 2.2. So maybe it's just a matter of not forcing the option here?