bug#32022: (new feature) Change guix pull to choose commits for which substitutes is already built by default

[swedebugia]

Hi

I would like it to be easy for newcomers to do the right thing when pulling.

I suggest we by default give guix pullers a bit more information and interaction.
Something like this:
"Checking for newly built derivations of guix at hydra.gnu.org...
(table)
Date            Commit         Succeeded for x64?
28/6            54d84d8        yes
... 
... 
... 

Pulling based on default commit selection scheme (see --help for details) 
Most recently built commit  is x4678x85 built x hours ago. 
Continuing in 10 seconds...
Press ctrl+c to abort. "

Guix pull gets a new argument " -d" allowing you to specify the days old commit you want to select. 
E. G. Guix pull -d 5 selects a commit build successfully 5 days ago by hydra. 

Guix weather could show the table only, if the argument --guix-pull was given. 
-- 
Cheers Swedebugia