Error when running `guix system reconfigure`: Read-only file system

[Tanguy Le Carrour]

Dear Guix,

I would have preferred to write to tell you that I had installed Guix on
all my computers (work, home and laptop) and that I was happily working
on packaging some Python and Rust libraries (for [fd][]), but…

[fd]: https://github.com/sharkdp/fd

I took 2 weeks off, and when I came back, the first thing I did was updating
my system (at work… I haven't tried at home yet) and ended up with this error:

```
❯ sudo guix system reconfigure ~/config/rafflesia.config.scm
The following derivation will be built:
   /gnu/store/ak375xdnkalqnvfardp9hwasb8wg440h-grub.cfg.drv
building /gnu/store/ak375xdnkalqnvfardp9hwasb8wg440h-grub.cfg.drv...
/gnu/store/92nna44aa0k2ppwx79bkn7rcq0c8br0y-system
/gnu/store/rzcb902axrd5cxxywxkcs7y5kh4z9hf3-grub.cfg

activating system...
making '/gnu/store/92nna44aa0k2ppwx79bkn7rcq0c8br0y-system' the current 
system...
setting up setuid programs in '/run/setuid-programs'...
populating /etc from /gnu/store/i0271lgj9wi7gzghv1zaiwp04gzfyfpj-etc...
guix system: error: mkdir: Read-only file system
```

But the file system doesn't look read-only to me:

```
/dev/sda3 on / type ext4 (rw,relatime)
```

Is it something I've done?! I'll try at home at the week end, just to
make sure.
Anyway, this won't keep me from working on my packaging! :-)

Any help welcome!

-- 
Tanguy