Add git repo checkout to testing workflow

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes #14634

.github/workflows/zfs-linux-tests.yml

@@ -13,6 +13,9 @@ jobs:
   zloop:
     runs-on: ubuntu-${{ inputs.os }}
     steps:
+    - uses: actions/checkout@v3
+      with:
+        ref: ${{ github.event.pull_request.head.sha }}
     - uses: actions/download-artifact@v3
       with:
         name: modules-${{ inputs.os }}
@@ -50,6 +53,9 @@ jobs:
   sanity:
     runs-on: ubuntu-${{ inputs.os }}
     steps:
+    - uses: actions/checkout@v3
+      with:
+        ref: ${{ github.event.pull_request.head.sha }}
     - uses: actions/download-artifact@v3
       with:
         name: modules-${{ inputs.os }}
@@ -83,6 +89,9 @@ jobs:
       matrix:
         tests: [ part1, part2, part3, part4 ]
     steps:
+    - uses: actions/checkout@v3
+      with:
+        ref: ${{ github.event.pull_request.head.sha }}
     - uses: actions/download-artifact@v3
       with:
         name: modules-${{ inputs.os }}