`${SOME_VAR%pattern}` was turning into `SOME_VAL%pattern}` which the shell would then balk at.
I've updated the `TOKEN_ENV_INTERPOLATION` regex to account for this (ie, if `${` 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. 😄
Signed-off-by: Andrew Page <admwiggin@gmail.com>
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