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

This reverts commit d2053309deff240513e630c13de1c996ea1c715e.
Merge pull request #8632 from dhylands/bug-847830-backout-v1-train

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