[cmake] Fix git short length

Summary: The short hash length vary with git versions, so let's fix it to have the same deterministic output on every machines.

Test Plan:

./contrib/guix/guix-clean && ./contrib/guix/guix-build

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D15651