Revert "Bug 847830 - Changed CUSTOMIZE environment variable to GAIA_DISTRIBUTION_DIR"

This reverts commit 8cfeec635cbbe6f3fc57e08812c87e7c14b3c3be.
Merge pull request #8639 from dhylands/bug-847830-backout-master

Revert "Bug 847830 - Changed CUSTOMIZE environment variable to GAIA_DIST...