[CI] Automated commits: fix ninja not running from the build directory

Summary:
This was missed during testing because I tested the script from a build
directory. The CI doesn't, which causes ninja to fail due to missing
ninja.build file.

Test Plan:

cd contrib/source-control-tools
COMMIT_TYPE=archive-release-notes ./automated-commits.sh

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7379