get Racket as source distribution instead of from GitHub repo

A source distribution for a release is configured with a suitable
package catalog, which avoids the problem reported as Racket PR 14648
(query at http://bugs.racket-lang.org/).

Using the "builtpkgs" variant of the source distribution makes the
installation work with pre-built packages, which enables faster installs
of Racket packages.
Need to add `version "6.0.1"` to the formula.