[guix] Work around a buggy bug detection

Summary:
aws-lc-sys is a crypto library that will be used to implement the tls
json server, and is a dependency for the karyon lib we use.
As a crypto lib, they detect a nasty bug that affected gcc versions 9
and 10 and caused memcmp to not work. Unfortunately the detection seems
to be broken when their cmake build system is not used. This diff
instructs the lib to use that non-bogus cmake build by setting the
appropriated environment variable.

Test Plan: Run the linux guix build.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17320