New 'version-1.3.0' branch and lifting string freeze on master

[Maxim Cournoyer]

Hello!

With the remaining issues to be tackled for the current release (v1.3.0)
due tomorrow, I think we may need a bit of extra time to fix them and do
more testing.  These are the remaining issues (obtained by pressing the
'b' key in Emacs Debbugs while visiting the parent issue with
'debbugs-gnu-bugs RET 47297 RET').  The release task issue can also be
viewed at https://issues.guix.gnu.org/47297.

47808 important  Bone Baboon        guile-git-0.5.0.drv build failed on 
i686-linux
47567 important  Alexandru-Sergiu M Installer crash in 'uuid->string' for a 
FAT16 partition
44872 important  Tim Magee          GuixSD 1.2.0 installer fails with exception 
when formatting drive
33848 important  Ludovic Courtès    Store references in SBCL-compiled code are 
"invisible"
47841 normal     Julien Lepiller    [release 1.2.1] could not install on 
foreign distro
47745 normal     Mathieu Othacehe   ldap test is failing
47744 normal     Mathieu Othacehe   nfs-root-fs test is failing

To avoid keep master in string freeze longer, I've now created the
'release-v1.3.0' branch where the fixes for the remaining blocking
issues should go.

I'll now attempt to produce a first release candidate (RC) via the
release tooling.

Thanks,

Maxim