Fix builder from being over-aggressive on ${}

`${SOME_VAR%pattern}` was turning into `SOME_VAL%pattern}` which the shell would then balk at.

I've updated the `TOKEN_ENV_INTERPOLATION` regex to account for this (ie, if `${` is used, it _must_ also match the closing `}`), and renamed the variable to not be exported (since it's not used outside the function following it).

I also added comments for the bits of `tokenEnvInterpolation` so they're easier to follow. :smile:
ping @erikh @jfrazelle 

care to take a look at these regex changes?