ci(kernel-rc-cross-os): pin actions to full commit SHAs (org policy)

databricks org requires all actions pinned to a full-length commit SHA;
actions/checkout@v4 and setup-node@v4 were rejected at 'Set up job'. Pin to
the same SHAs main.yml uses.

Co-authored-by: Isaac

Author: msrathore-db

.github/workflows/kernel-rc-cross-os.yml

@@ -61,8 +61,8 @@ jobs:
           - { label: darwin-arm64,    os: macos-latest,     node: 20 }
           - { label: win32-x64-msvc,  os: windows-latest,   node: 20 }
     steps:
-      - uses: actions/checkout@v4
-      - uses: actions/setup-node@v4
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
+      - uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4
         with:
           node-version: ${{ matrix.node }}
       - name: Platform info
@@ -99,7 +99,7 @@ jobs:
           - { label: linux-x64-musl,   runner: ubuntu-latest,    image: 'node:16-alpine', node16: 'yes' }
           - { label: linux-arm64-musl, runner: ubuntu-24.04-arm, image: 'node:20-alpine', node16: '' }   # needs ARM runner availability
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
       - name: Run suite in ${{ matrix.image }}
         working-directory: .
         run: |