ci: bump actions/checkout v4 -> v5 (clear Node 20 deprecation) (#176)

actions/checkout@v4 runs on the deprecated Node 20 runtime, which GitHub
now force-runs on Node 24 and warns about on every job. v5 targets Node
24 natively. Bump all uses in the tracked workflows (release.yml,
pr-build-check.yml).

Third-party actions (svenstaro/upload-release-action, tpaschalis/
s3-sync-action, appleboy/ssh-action) are left as-is; they are not the
actions/* Node-20 source flagged here.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: widgetii

.github/workflows/pr-build-check.yml

@@ -27,7 +27,7 @@ jobs:
             toolchain_dir: aarch64--musl--stable-2025.08-1
             cc: aarch64-buildroot-linux-musl-gcc
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
       - name: Fetch toolchain
         run: |
           # Download to file (with retries) before extracting — piping
@@ -49,6 +49,6 @@ jobs:
     name: Test sensor extraction pipeline
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
       - name: Run end-to-end pipeline smoke test
         run: tools/test_pipeline.sh

.github/workflows/release.yml

@@ -27,7 +27,7 @@ jobs:
       branch_name: ${{ steps.vars.outputs.branch_name }}
       head_tag: ${{ steps.vars.outputs.head_tag }}
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
         with:
           fetch-depth: 0
 
@@ -115,7 +115,7 @@ jobs:
       HEAD_TAG: ${{ needs.release.outputs.head_tag }}
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
         with:
           fetch-depth: 0