lock ubuntu version in GitHub actions

.github/workflows/lint.yml

@@ -10,7 +10,7 @@ jobs:
   golangci-lint:
     name: golangci-lint
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2
@@ -22,7 +22,7 @@ jobs:
   go-mod-tidy:
     name: go-mod-tidy
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2

.github/workflows/release.yml

@@ -9,7 +9,7 @@ jobs:
   github:
     name: github
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2
@@ -46,7 +46,7 @@ jobs:
   dockerhub:
     name: dockerhub
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2

.github/workflows/test.yml

@@ -10,7 +10,7 @@ jobs:
   test:
     name: test
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2
@@ -22,7 +22,7 @@ jobs:
   test32:
     name: test32
 
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
     - uses: actions/checkout@v2