Racket package: disable parallel build; add variants (#27506)
Racket package: disable parallel build; add variants (#27506)

- Prevent `-j` flags to `make`, which has been known to cause problems
  with Racket builds.
- Add variants for various common build flags, including support
  for both versions of the Racket VM environment.

In addition:

- Prefer the minimal release to improve install times. Bells and
  whistles carry their own runtime dependencies and should be
  installed via `raco`. An enterprising user may even create a
  `RacoPackage` class to make spack aware of `raco` installed packages.
- Match the official version numbering scheme.
LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack