Bugfix: Change ':=' to ':-'

I was confused as to the difference between `:=` and `:-`.  `:=` is more like
Ruby's `||=` operator.  This is what we typically want, however, due to all of
the lines already having assignment with an `export` directive, I chose to
convert them to `:-` for now just to fix the bug.

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Signed-off-by: Jeff Felchner <accounts+git@thekompanee.com>