bug#62051: Early detection of derivations with unreadable builder scripts

[Christopher Baines <mail@cbaines.net>]

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Currently it's quite easy to end up with packages that have builder
scripts that can't be read by Guile.

This is part of the following builder script:

  (cons "--enable-mpi-java" #<gexp  gnu/packages/mpi.scm:233:24 7f366e0cd930>)

from: /gnu/store/yngxnpcs4s6y8acxf4nwx5pcpj0j6q6i-java-openmpi-4.1.4-builder

And when attempting to build that derivation, you get the following
error.

  ice-9/read.scm:126:4: In procedure read-expr*:
  /gnu/store/yngxnpcs4s6y8acxf4nwx5pcpj0j6q6i-java-openmpi-4.1.4-builder:1:3820: Unknown # object: "#<"


It would be nice if Guix could detect this category of problems and
raise an error at the time the derivation is created, rather than the
error occuring only when you build the derivation.

This would be helpful particularly for the Guix Data Service since
currently it ends up storing these useless derivations, often many times
since the builder includes some often changing string (7f366e0cd930 in
the example above), so this is a common cause of spurious changes
between revisions (as often noted on qa.guix.gnu.org).

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