fix: making builds of guix to produce same output whatever git config

[knst]

Builds are different, because git command output are different:

Issue being fixed or feature implemented

in pr #5194 different OS produced different hashes of binary.
That's due to different behaviour of this command:

$ git rev-parse --short HEAD
b3f242da14 - on my localhost Kubuntu 22.10
b3f242da1 - inside pastapastapasta's docker

What was done?

This fix forces to git print always 12 character

How Has This Been Tested?

Run in different environment

Breaking Changes

No breaking changes

Checklist:

• I have performed a self-review of my own code
• I have assigned this pull request to a milestone

[knst]

$ git log --oneline | head -n 1
4c57f4dc72 fix: making builds of guix to produce same output whatever git config
$ sha1sum distsrc-x86_64-linux-gnu/src/dashd
401be366c0fbde3f4b054280558f48d74bfa56d6  distsrc-x86_64-linux-gnu/src/dashd

[UdjinM6]

401be366c0fbde3f4b054280558f48d74bfa56d6  distsrc-x86_64-linux-gnu/src/dashd

ACK

[PastaPastaPasta]

utACK for squash merge