more than one target service shehperd-root / possible scheme records terribleness

[raingloom]

I have an operating-system called bingobongo. I have another called
greg that inherits the bingobongo config. If I leave the services field
empty in greg, there is no issue. If I define it as
(operating-system-services bingobongo) then all hell breaks loose and I
get the aforementioned error and no amount of filtering seems to fix it
becaues then I get the same error but with "profile" instead of
shepherd-root.

Now, I don't know what causes this, whether it's a Guix issue or
Guile's record system being... well... kind of terrible, but in any
case it should be treated like a bug, because a user should not have to
dig as deeply into Guix internals as I have so far.