bug#64856: guix shell cache doesn't consider grafts

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Hello,

While investigating https://issues.guix.gnu.org/64836, I discovered that
the cache of 'guix shell' doesn't take into account grafts for
subsequent invocations.

Consider, using Guix at commit 21b718f4d6c3ded8ef50d12f6e9ae6474f74620f:

--8<---------------cut here---------------start------------->8---
$ guix build gtk+
/gnu/store/6vqx7nip7r95h2nhvhb9vxzjpri581b9-gtk+-3.24.37

$ guix build --no-grafts gtk+
/gnu/store/2n2kprz35a19ibs5kbjsb3k4cdl69q2w-gtk+-3.24.37

$ guix shell gtk+ -- sh -c 'realpath $GUIX_ENVIRONMENT/lib/libgtk-3.so'
/gnu/store/6vqx7nip7r95h2nhvhb9vxzjpri581b9-gtk+-3.24.37/lib/libgtk-3.so.0.2405.32

$ guix shell --no-grafts gtk+ -- sh -c 'realpath $GUIX_ENVIRONMENT/lib/libgtk-3.so'
/gnu/store/6vqx7nip7r95h2nhvhb9vxzjpri581b9-gtk+-3.24.37/lib/libgtk-3.so.0.2405.32
--8<---------------cut here---------------end--------------->8---

The 'guix shell --no-grafts' invocation simply reused the same cache as
the previous command, so grafts are in use, which is non-intuitive.  The
reverse would be true as well:

--8<---------------cut here---------------start------------->8---
$ guix shell --rebuild-cache --no-grafts gtk+ -- sh -c 'realpath $GUIX_ENVIRONMENT/lib/libgtk-3.so'
/gnu/store/2n2kprz35a19ibs5kbjsb3k4cdl69q2w-gtk+-3.24.37/lib/libgtk-3.so.0.2405.32

$ guix shell gtk+ -- sh -c 'realpath $GUIX_ENVIRONMENT/lib/libgtk-3.so'
/gnu/store/2n2kprz35a19ibs5kbjsb3k4cdl69q2w-gtk+-3.24.37/lib/libgtk-3.so.0.2405.32
--8<---------------cut here---------------end--------------->8---

The ungrafted cache got reused by the invocation that should have used
grafted inputs.

-- 
Thanks,
Maxim