use ubuntu-18.04 in ci (#883)

.github/workflows/build.yml

@@ -7,7 +7,7 @@ env:
 
 jobs:
   lint:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v2
       - name: Set up Python 3.7
@@ -28,7 +28,7 @@ jobs:
           style: google
 
   build_without_torch:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     strategy:
       matrix:
         python-version: [3.7]
@@ -51,7 +51,7 @@ jobs:
           pytest tests/ --ignore=tests/test_runner --ignore=tests/test_optimizer.py --ignore=tests/test_cnn --ignore=tests/test_parallel.py --ignore=tests/test_ops --ignore=tests/test_load_model_zoo.py --ignore=tests/test_utils/test_logging.py --ignore=tests/test_image/test_io.py --ignore=tests/test_utils/test_registry.py --ignore=tests/test_utils/test_parrots_jit.py
 
   build_without_ops:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     env:
       MMCV_WITH_OPS: 0
     strategy:
@@ -81,7 +81,7 @@ jobs:
           pytest tests/ --ignore=tests/test_ops
 
   build_cpu:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     strategy:
       matrix:
         python-version: [3.7]
@@ -122,7 +122,7 @@ jobs:
           coverage report -m
 
   build_cuda:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     env:
       CUDA: 10.1.105-1
       CUDA_SHORT: 10.1

.github/workflows/build_pat.yml

@@ -7,7 +7,7 @@ env:
 
 jobs:
   build_parrots:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     container:
       image: ghcr.io/sunnyxiaohu/parrots-mmcv:1.2.1
       credentials:

.github/workflows/publish-to-pypi.yml

@@ -4,7 +4,7 @@ on: push
 
 jobs:
   build-n-publish:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     if: startsWith(github.event.ref, 'refs/tags')
     steps:
       - uses: actions/checkout@v2
@@ -20,7 +20,7 @@ jobs:
           twine upload dist/* -u __token__ -p ${{ secrets.pypi_password }}
 
   build-n-publish_with_ops:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     if: startsWith(github.event.ref, 'refs/tags')
     steps:
       - uses: actions/checkout@v2