bug#56722: guix pull fails -- unbound variable in gnu/home/services/symlink-manager.scm

[(]

Log here:
<https://paste.sr.ht/~unmatched-paren/201220d944b25ae9074d94c29b88852de70730cb>

What's weird is that the culprit, gnu/home/services/symlink-manager.scm,
*does* have access to `service-type`, because it imports (gnu home
services), which re-exports it!

My attempts to `git bisect` were thwarted by another build failure:

```
[I] /home/paren/code/guix
ʃ git bisect start
[I] /home/paren/code/guix
ʃ git bisect bad
[I] /home/paren/code/guix
  # the commit I'm on
ʃ git bisect good cd3c4ad8fc113608eade2e5c29ba8486cde01157
[I] /home/paren/code/guix
ʃ git bisect run guix shell -D guix -- make
[log output: 
<https://paste.sr.ht/~unmatched-paren/af420655dffb4f0af2b36a014725602a102989f2>]
```

This bug is also experienced by `vhallac` on IRC.

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