Run GH actions on ubuntu-20.04 (#18)

/cherry-pick

Signed-off-by: 1gtm <1gtm@appscode.com>

Signed-off-by: 1gtm <1gtm@appscode.com>

Author: 1gtm

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ concurrency:
 jobs:
   build:
     name: Build
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
 
       - name: Set up Go 1.19

.github/workflows/release-tracker.yml

@@ -12,7 +12,7 @@ concurrency:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
 
     steps:
       - uses: actions/checkout@v1

.github/workflows/release.yml

@@ -14,7 +14,7 @@ concurrency:
 jobs:
   build:
     name: Build
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v1