Fix build agent tag (#1821)

Author: yongtang

.github/workflows/build.yml

@@ -450,7 +450,7 @@ jobs:
     name: Release
     if: github.event_name == 'push'
     needs: [linux-wheel, macos-wheel] #, windows-wheel]
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/download-artifact@v1
         with:
@@ -541,7 +541,7 @@ jobs:
     name: Release Candidate
     if: github.event_name == 'push' && github.ref == 'refs/heads/master'
     needs: [lint, release]
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
       - uses: actions/download-artifact@v1
@@ -581,7 +581,7 @@ jobs:
     name: Docker Release
     if: github.event_name == 'push' && github.ref == 'refs/heads/master'
     needs: [lint, linux-test, macos-test, windows-test]
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v1
@@ -741,7 +741,7 @@ jobs:
     name: Nightly
     if: github.event_name == 'push'
     needs: [linux-nightly, macos-nightly, windows-nightly]
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/download-artifact@v1
         with:
@@ -857,7 +857,7 @@ jobs:
     name: Docker Nightly
     if: github.event_name == 'push' && github.ref == 'refs/heads/master'
     needs: [linux-nightly, macos-nightly, windows-nightly]
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
       - uses: actions/setup-python@v1