bug#49100: make check fails: %derivation-cache

[Chris Marusich <cmmarusich@gmail.com>]

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Hi,

On core-updates (a6c292a6f123acc86429722619ccb51ca54f844f), "make check"
errors out in tests/builders.scm:

--8<---------------cut here---------------start------------->8---
Backtrace:
           1 (primitive-load-path "tests/builders.scm")
In guix/tests.scm:
    146:8  0 (call-with-external-store #<procedure 7f92ff4f7d60 at i…>)

guix/tests.scm:146:8: In procedure call-with-external-store:
error: %derivation-cache: unbound variable
--8<---------------cut here---------------end--------------->8---

The problem appears to have been caused by
7d873f194ca69d6096d28d7a224ab78e83e34fe1 ("build-system: Rewrite using
gexps.").

The attached patch fixes the issue for me.  However, since I'm not sure
how %derivation-cache is or was supposed to be used, I would appreciate
a second opinion.

Note that %derivation-cache has been used to refer to two different
things in the past (see: 3182539875a67f5989c73c3c654fe3138bbc275c).
Note also that even after applying this fix, some tests relying on
call-with-external-store still fail when run (see: bug 47018).

-- 
Chris

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