.github/zephyr: test pull/12345/head instead of pull/12345/merge

.github/zephyr: test pull/12345/head instead of pull/12345/merge

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

.github/workflows/zephyr.yml

@@ -129,10 +129,15 @@ jobs:
         # build reproducibility. sof.git is still small.
         # This is especially useful for daily builds (but not just).
         with:
-          fetch-depth: 0
+          fetch-depth: 5
           filter: 'tree:0'
           path: ./workspace/sof
 
+      # pull/1234/merge is tested by default. This time we want
+      # pull/1234/head instead.
+      # https://github.com/actions/checkout
+      - run: git -C workspace/sof checkout HEAD^2
+
       - name: west clones
 
         run: pip3 install west && cd workspace/sof/ && west init -l &&
@@ -230,10 +235,15 @@ jobs:
     steps:
       - uses: actions/checkout@v4
         with:
-          fetch-depth: 0
+          fetch-depth: 5
           filter: 'tree:0'
           path: ./workspace/sof
 
+      # pull/1234/merge is tested by default. This time we want
+      # pull/1234/head instead.
+      # https://github.com/actions/checkout
+      - run: git -C workspace/sof checkout HEAD^2
+
       # Cache artifacts so we do not overload external servers with downloads
       # Remember to change step key if you change the tools so old cache is not restored,
       # or delete cache manually in Github Actions tab