Substituting package definition for dependency

[Paul Jewell]

thermald doesn't build at the moment (bug 54061). Maxime has created 
some patches which get around the problem, something I confirmed by 
building from a local patched git repo. I would like to incorporate 
these patches so I can continue to update guix, until such time the 
patches find their way into the main repo. Is there some way I can do 
this? I know I can provide a different definition for a package I 
explicitly want to build, but how can I do this for a package built as a 
dependency during a "guix system reconfigure"?

Alternatively, should I be using my git clone to build my system instead 
of guix pull?

Apologies if this is clearly explained in the documentation - I couldn't 
find anything to help. If I have missed something, a pointer would be 
helpful!

Thanks!