The first revision of this patch does not take care to avoid changing
the derivation of xorg-server-for-tests, so it would cause way too many
packages to be rebuilt.

Here is a revised patch that ensures the derivation remains the same.

For example:

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$ guix build -e '(@@ (gnu packages xorg) xorg-server-for-tests)' --no-grafts -d                  
/gnu/store/nhs1c9q04g6k4prxxv4kb9q5lg1p872q-xorg-server-1.20.10.drv
$ ./pre-inst-env guix build -e '(@@ (gnu packages xorg) xorg-server-for-tests)' --no-grafts -d
/gnu/store/nhs1c9q04g6k4prxxv4kb9q5lg1p872q-xorg-server-1.20.10.drv
------