.github/zephyr: re-add TGL IPC3 build for now

Partial revert of commit 687c6f3 (".github: workflow: removed
legacy RTOS platforms from Zephyr build") that was a bit too
"enthousiastic".

When someone breaks the Zephyr+IPC3 build we'd like to: 1. notice, 2.
tell whether it's Intel-specific, IMX-specific or not specific.

This is a one-line change and unlike testing, building is "free".
It can be removed in a second when/if that becomes a burden.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

Author: marc-hb

.github/workflows/zephyr.yml

@@ -19,6 +19,7 @@ jobs:
         IPC_platforms: [
           # - IPC3 default
           imx8 imx8x imx8m,
+          tgl tgl-h,  # UNSUPPORTED! Will be removed
           # - IPC4 default
           mtl,
           # Temporary testbed for Zephyr development.