Bump GitHub Actions to v5 to resolve Node.js 20 deprecation

Update actions/checkout v4→v5 and actions/setup-go v4→v5 across all
workflows (push-test, nightly, release) to use Node.js 24.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ako

.github/workflows/nightly.yml

@@ -13,11 +13,11 @@ jobs:
   nightly:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
         with:
           fetch-depth: 0
 
-      - uses: actions/setup-go@v4
+      - uses: actions/setup-go@v5
         with:
           go-version: '1.26'
 

.github/workflows/push-test.yml

@@ -6,8 +6,8 @@ jobs:
   build-and-test:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
-      - uses: actions/setup-go@v4
+      - uses: actions/checkout@v5
+      - uses: actions/setup-go@v5
         with:
           go-version: '1.26'
       - name: Build

.github/workflows/release.yml

@@ -12,10 +12,10 @@ jobs:
   release:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
         with:
           fetch-depth: 0
-      - uses: actions/setup-go@v4
+      - uses: actions/setup-go@v5
         with:
           go-version: '1.26'
       - name: Build release binaries