.github: add a test for xtensa-build-zephyr.py --all

Add a test to avoid --all breakages like the one in commit
05bfc36 ("ptl: Add initial PTL configuration"), reported in thesofproject#9262

See also previous commit.

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

.github/workflows/zephyr.yml

@@ -123,6 +123,11 @@ jobs:
             zephyr_revision: mnfst
             IPC_platforms: mtl
 
+          # This is "duplication of effort" but it makes sure no one
+          # breaks --all, see for instance #9292 and previous commit.
+          - zephyr_revision: mnfst
+            IPC_platforms: --all
+
     steps:
       - uses: actions/checkout@v4
         # Download a full clone to fix `git describe`, sof_version.h and