Is `guix gc --references FOO` expected to require a writable store?

[Leo Famulari]

On #guix, aure84 mentioned that they aren't able to use `guix gc
--references` on immutable VMs created with `guix system vm`.

I was a little suprised, not expecting this command to require any
writes to the store.

For example, I create and run a VM from the bare-bones template:

`guix system vm --no-grafts doc/os-config-bare-bones.texi`

Within the VM, I try the command:

------
# guix gc --references 
/gnu/store/cbzabyh8kdks7ip1hk4cycldq1wiflg3-automake-skip-amhello-tests.patch
guix gc: error: changing ownership of path '/gnu/store': Read-only file system
------

Is this expected? Is the error message even correct, or is it
misleading?

BTW, I ran the command in `strace -f`. The log is attached.

log
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