gexp: 'compiled-modules' no longer overrides (guix build utils).

Until now 'compiled-modules' would override (guix build utils) with its
own.  Thus, when asked to build a different (guix build utils),
via #:module-path, it would fail badly because a (guix build utils)
module was already loaded and possibly incompatible with the new one.

This happened when running 'guix pull --branch=core-updates' from
current master: in 'core-updates', (guix build utils) exports
'ignore-error?' but in 'master' it does not.  Thus, 'guix pull' would
fail with:

  no binding `invoke-error?' in module (guix build utils)
  builder for `/gnu/store/…-module-import-compiled.drv' failed with exit code 1
  cannot build derivation `/gnu/store/…-compute-guix-derivation.drv': 1 dependencies couldn't be built

This patch fixes it.

* guix/gexp.scm (compiled-modules)[build-utils-hack?]: New variable.
[build]: Load MODULES/build/utils.scm when it exists.

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