build: ruby: Avoid long build directory names.

Having the hash of the source gem in the source directory file name proved to
be problematic when running the test suite for the 'pg' gem that creates
UNIX-domain sockets in the source directory and exceeded the 108 character
limit on GNU/Linux systems.

* guix/build/ruby-build-system.scm (unpack): Rename unpacked gem directory to
  "gem".