bug#70021: 6f9d844 causes system hang right before GDM supposed to start

[aurtzy <aurtzy@gmail.com>]

Hi!

I recently encountered an issue with my system hanging during the 
startup process, where it halts just before GDM is supposed to start.

Bisect has led to commit 6f9d844 (related issue: 
https://issues.guix.gnu.org/67649 ) being the culprit.

Context/workaround:

This appears to be an issue with code relating to mapped drives. Further 
inspection reveals my `device-mapping-cryptstorage` service fails to 
start, which is supposed to map my secondary encrypted drive. I 
currently work around this by commenting out the `file-system` entry 
that depends on this mapping before mounting it.

I managed to find the following line in /var/log/messages, which - if I 
understand the change correctly - points to `bytevector?` being now 
unavailable, causing the device mapping service to fail (and take the 
system with it?):

Mar 26 17:50:27 localhost vmunix: [   24.680761] shepherd[1]: Exception 
caught while starting device-mapping-cryptstorage: (unbound-variable #f 
"Unbound variable: ~S" (bytevector?) #f)

Best,

aurtzy