workflows: iot-gate-imx8plus: add custom template path

[acostach]

A custom template path has originally been added in #3599 for the iot-gate-imx8. The iot-gate-imx8plus needs one as well because it's now using the meta-balena-hab submodule.

Change-type: patch

Needed for merging #3602

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• Changes have been tested
  • Covered in automated test suite
  • Manual test case recorded
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