.github/zephyr: de-hardcode the name of the zephyr remote

Fixes issue reported in stable-v2.7 thesofproject#8353 which wants to use our own,
downstream Zephyr branch.

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

Author: marc-hb

.github/workflows/zephyr.yml

@@ -167,7 +167,7 @@ jobs:
           #     both issues in no time.
 
           cd zephyr
-          git fetch --filter=tree:0 zephyrproject "$rem_rev":_branch_placeholder
+          git fetch --filter=tree:0 "$(git remote |head -n1)" "$rem_rev":_branch_placeholder
           git branch -D _branch_placeholder
 
           set -x