Upgrade actions/checkout from v4 to v6 (#683)

## Summary
- Upgrade all `actions/checkout` references from v4 to v6 in CI workflow

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: dharmab

.github/workflows/skyeye.yaml

@@ -15,7 +15,7 @@ jobs:
       contents: read
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Setup
         uses: ./.github/actions/setup
       - name: Build whisper.cpp
@@ -35,7 +35,7 @@ jobs:
       contents: read
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Setup
         uses: ./.github/actions/setup
       - name: Build whisper.cpp
@@ -49,7 +49,7 @@ jobs:
       contents: read
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Setup
         uses: ./.github/actions/setup
       - name: Build whisper.cpp
@@ -86,7 +86,7 @@ jobs:
       contents: read
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Install dependencies
         shell: bash
         run: make install-macos-dependencies
@@ -129,7 +129,7 @@ jobs:
       GOROOT: /ucrt64/lib/go
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Set up MSYS2
         uses: msys2/setup-msys2@v2
         with:
@@ -190,7 +190,7 @@ jobs:
       contents: read
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Build image
         uses: docker/build-push-action@v6
         with:
@@ -210,7 +210,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Download artifacts
         uses: actions/download-artifact@v4
         with:
@@ -239,7 +239,7 @@ jobs:
       id-token: write
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
       - name: Build and push image
         uses: ./.github/actions/build-container
         with: