test: update ubuntu-18.04 to ubuntu-latest in GitHub workflows

Author: andyzhangx

.github/workflows/static.yaml

@@ -5,7 +5,7 @@ on:
 jobs:
     go_lint:
         name: Go Lint
-        runs-on: ubuntu-18.04
+        runs-on: ubuntu-latest
         steps:
             - name: Checkout code
               uses: actions/checkout@master

.github/workflows/trivy.yaml

@@ -7,7 +7,7 @@ on:
 jobs:
   build:
     name: Build
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
       - name: Set up Go 1.x
         uses: actions/setup-go@v2