[CI] Automated commits: use nameref to get the current version

Summary:
Using the standard output to get the version cause garbage to get into
the variable. Using nameref ensure only the version string is assigned
to the named variable.

Test Plan:

COMMIT_TYPE=update-aur-version-bitcoin-abc \
  ./contrib/source-control-tools/automated-commits.sh

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7390