ruby-build-system: Add wrap-ruby-program.

A modified copy of wrap-program from (guix build utils). The wrap-program
procedure doesn't work well for Ruby scripts, as it breaks using the -S flag
with ruby to execute the script, as when -S is passed to ruby, it expects the
script on the PATH to use ruby in the shebang, and not bash.

Therefore, to wrap the program, but keep the shebang as ruby, wrap it with a
ruby script instead.

wrap-ruby-program uses .real/foo rather than .foo-real, as this might be
neater. This procedure also includes a call to Gem.clear_paths to make it
possible to set the GEM_PATH through this method, and for it to take effect.

* gnu/build/ruby-build-system.scm (wrap-ruby-program): New procedure.