Have single source of truth for value of BUILD_DIR

[cpcallen]

At the moment, BUILD_DIR is set in scripts/gulpfiles/config.js, but certain bash scripts in tests/scripts/ also need to know this value and for now have hardcoded constants.

Describe the solution you'd like

Have the bash scripts read the value of BUILD_DIR from config.js.

Describe alternatives you've considered

Have config.js (and the scripts) read the value of BUILD_DIR from some other more bash-compatible config file, or maybe an environment variable.

Additional context

[rachel-fenichel]

@cpcallen Does #6403 include this work?

[cpcallen]

@cpcallen Does #6403 include this work?

No. In fact:

• Some of the places which presently reference build/ (BUILD_DIR) will in fact need to point at dist/ (RELEASE_DIR) instead.
• Hardcoding references to these directories seems undesirable (hence this bug), but I now realise that the alternative is hardcoding references to scripts/gulpfiles/config.js (or some equivalent) which isn't obviously better.

I will think further about this matter and may close this bug.