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

This reverts commit e798c71ea1a8e1e5407ec70ba6469635a7e4e6bb.
Merge pull request #8631 from dhylands/bug-847830-backout-v1.0.1

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