fix: Support array format for runs-on field in GitHub Actions workflows

- Add custom deserializer for runs-on field to handle both string and array formats
- Update Job struct to use Vec<String> instead of String for runs-on field
- Modify executor to extract first element from runs-on array for runner selection
- Add test workflow to verify both string and array formats work correctly
- Maintain backwards compatibility with existing string-based workflows

Fixes issue where workflows with runs-on: [self-hosted, ubuntu, small] format
would fail with 'invalid type: sequence, expected a string' error.

This change aligns with GitHub Actions specification which supports:
- String format: runs-on: ubuntu-latest
- Array format: runs-on: [self-hosted, ubuntu, small]

tests/workflows/runs-on-array-test.yml

@@ -0,0 +1,18 @@
+name: Test Runs-On Array Format
+
+on: [push]
+
+jobs:
+  test-array-runs-on:
+    timeout-minutes: 15
+    runs-on: [self-hosted, ubuntu, small]
+    steps:
+      - name: Test step
+        run: echo "Testing array format for runs-on"
+
+  test-string-runs-on:
+    timeout-minutes: 15
+    runs-on: ubuntu-latest
+    steps:
+      - name: Test step
+        run: echo "Testing string format for runs-on"