build-script-helper.py needs to support Python3 to support Ubuntu 22.04

[shahmishal]

/home/build-user/indexstore-db/Utilities/build-script-helper.py build --package-path /home/build-user/indexstore-db --build-path /home/build-user/build/buildbot_linux/indexstoredb-linux-x86_64 --configuration release --toolchain /home/build-user/swift-nightly-install/usr --ninja-bin /home/build-user/build/buildbot_linux/ninja-build/ninja
/usr/bin/env: ‘python’: No such file or directory

https://ci.swift.org/job/oss-swift-package-ubuntu-22_04/84/console

[ahoppen]

Fixed in #151.