bug#35268: guix system won't open config files in tmpfs

[rendaw <7e9wc56emjakcm@s.rendaw.me>]

Package: guix
Version: 0.16.0

As a workaround for the "guix system init -e" issue I'm creating a
temporary file with the -e expression and passing it to guix like:

```
$ guix system init /tmp/tmpmf8u61uc.scm temp/mount
...
guix system: error: failed to load '/tmp/tmpmf8u61uc.scm': No such file
or directory
```

I also tried it with a relative path in case the absolute path was the
issue, but no luck.  The command works if I create the tmpfile in my
current (ext4) directory.  The permissions on the file are the same in
both cases (user:rw only).  /tmp is a tmpfs mount owned by user/group
root and has full (ugo:rwx) access.

I think there's two issues here:
1. For some reason files in tmpfs don't work
2. The error message is misleading