Vendor Ruby if an installation is not found

[edmorley]

This buildpack requires that a Ruby installation be available during buildpack compile, in order to install the parseconfig gem, and then run the install_git_submodules.rb script.

On Heroku-22 the stack image no longer includes a system Ruby installation, so in order for this buildpack to continue to work, a Ruby install must be vendored by this buildpack.

If an app already uses the Ruby buildpack, and that buildpack is ordered prior to the submodules buildpack, then this buildpack will skip the Ruby vendoring step to save downloading a redundant copy of Ruby.

The Ruby installation vendored by this buildpack is not made available to later buildpacks or at runtime, so will otherwise not affect the build.

The implementation is based on that in heroku/heroku-buildpack-nginx#103 (except for that buildpack, Ruby needed to be available at runtime, whereas here it is only needed during the build).

Fixes #7.

[edmorley]

Test run on Heroku-22 (where Ruby installation needed):

-----> Building on the Heroku-22 stack
-----> Using buildpack: https://github.com/edmorley/heroku-buildpack-submodules#vendor-ruby
-----> GIT Submodules app detected
-----> Ruby installation not found, temporarily installing Ruby for use by this buildpack
-----> Installed Ruby 3.1.2
-----> Installing parseconfig gem
Successfully installed parseconfig-1.1.2
Parsing documentation for parseconfig-1.1.2
Installing ri documentation for parseconfig-1.1.2
Done installing documentation for parseconfig after 0 seconds
1 gem installed
-----> Installing submodule libcnb.rs 
       Removing submodule git folder
-----> Discovering process types
       Procfile declares types -> (none)
-----> Compressing...
       Done: 84.9K

And on Heroku-20 (where system Ruby exists, so vendoring isn't needed):

-----> Building on the Heroku-20 stack
-----> Using buildpack: https://github.com/edmorley/heroku-buildpack-submodules#vendor-ruby
-----> GIT Submodules app detected
-----> Installing parseconfig gem
Successfully installed parseconfig-1.1.2
Parsing documentation for parseconfig-1.1.2
Installing ri documentation for parseconfig-1.1.2
Done installing documentation for parseconfig after 0 seconds
1 gem installed
-----> Installing submodule libcnb.rs 
       Removing submodule git folder
-----> Discovering process types
       Procfile declares types -> (none)
-----> Compressing...
       Done: 85.4K

(Both of these were using GitHub sync, since if git push is used, then the Heroku git server's built in submodules support takes over instead.)

[dmathieu]

Thank you Ed! ❤️

[edmorley]

Thank you for the quick review/merge! :-)