plt-racket: use source distribution.

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.
plt-racket: add 6.0.1 bottle.

Closes #31063.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>