Pin testing environment to ubuntu-18.04

Signed-off-by: RainbowMango <qdurenhongcai@gmail.com>

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ on:
 jobs:
   golangci:
     name: lint
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - name: checkout code
         uses: actions/checkout@v2
@@ -19,7 +19,7 @@ jobs:
         run: hack/verify-staticcheck.sh
   codegen:
     name: codegen
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - name: checkout code
         uses: actions/checkout@v2
@@ -34,7 +34,7 @@ jobs:
   build:
     name: compile
     needs: codegen # rely on codegen successful completion
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - name: checkout code
         uses: actions/checkout@v2
@@ -51,7 +51,7 @@ jobs:
   test:
     name: unit test
     needs: build
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - name: checkout code
         uses: actions/checkout@v2
@@ -63,10 +63,8 @@ jobs:
         run: make test
   e2e:
     name: e2e test
-    if: ${{ github.repository == 'karmada-io/karmada' }}
     needs: build
-    # switch back to GitHub Action after https://github.com/karmada-io/karmada/issues/481 get fixed.
-    runs-on: self-hosted
+    runs-on: ubuntu-18.04
     steps:
       - name: checkout code
         uses: actions/checkout@v2

.github/workflows/image.yml

@@ -10,7 +10,7 @@ jobs:
     # 1. running on the forked repository would fail as missing necessary secret.
     # 2. running on the forked repository would use unnecessary GitHub Action time.
     if: ${{ github.repository == 'karmada-io/karmada' && github.ref == 'refs/heads/master' }}
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - name: checkout code
         uses: actions/checkout@v2