Issue 53878, v3.
See <https://issues.guix.gnu.org/53878#72>.
Changes since v2:
* 05/15: Fix typo in commit message.
* 09/15: Add `%racket-version` and `%racket-origin`.
* 11/15: Add `%chez-scheme-for-racket-version`.
* 15/15: Use `%racket-version` and `%racket-origin`. Also:
* `racket-minimal`: The package to install explicitly is
"racket-lib" ("base" is implicit).
* `make-installation-layer.rkt`: Adjust workaround for
<https://github.com/racket/racket/issues/4133>, because `racket`
(but not `racket-minimal`) ends up actually creating the bogus
untethered "bin" directory.