[CI] Wrap the build_autotools.sh script in a function

Summary:
Similar to what is done with build_cmake.sh, it hides the script path
and environment expansion from the caller.

Depends on D6686.

Test Plan:

./build-configurations.sh build-autotools

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6687