[teamcity/build.sh] Read CONFIGURE_FLAGS from environment

Summary: This enables us to run non-default builds, such as ASAN, from TeamCity.

Test Plan:

# Default build as expected
./build.sh
# ASAN build
CONFIGURE_FLAGS="--with-sanitizers=address" ./build.sh

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3571