workflows: iot-gate-imx8: add custom template path

[alexgg]

With the addition of the meta-balena-hab submodule there are multiple balena layers with custom templates - specify which one to use to avoid a build error.

Changelog-entry: add custom template path to iot-gate-imx8

---

Contributor checklist

• Changes have been tested
  • Covered in automated test suite
  • Manual test case recorded
• Change-type present on at least one commit
• Signed-off-by is present
• The PR complies with the Open Embedded Commit Patch Message Guidelines

Reviewer Guidelines

• When submitting a review, please pick:
  • 'Approve' if this change would be acceptable in the codebase (even if there are minor or cosmetic tweaks that could be improved).
  • 'Request Changes' if this change would not be acceptable in our codebase (e.g. bugs, changes that will make development harder in future, security/performance issues, etc).
  • 'Comment' if you don't feel you have enough information to decide either way (e.g. if you have major questions, or you don't understand the context of the change sufficiently to fully review yourself, but want to make a comment)