fix: use proper name for GLTF support env variable

[yedpodtrzitko]

PR Goal

both docs and .travis.yml use GLTF_EXPERIMENTAL_SUPPORT_ENABLED as the expected name for env variable to enable GLTF support. The code itself however uses GLTF_EXPERIMENTAL_SUPPORT (missing ENABLED suffix).

This PR should fix the discrepancy.

Checkpoints

• [ ] If relevant, manual tests have been done to ensure the stability of the whole application and that the involved feature works
• [ ] Automated tests covering the feature or the fix, have been written, deemed irrelevant (give the reason), or an issue has been created to implement the test (give the link)
• [ ] The PR or the original issue contains a short summary of the implemented solution.

[yedpodtrzitko]

@Haskaris done

[Haskaris]

Everything is good, thank you for your contribution!