Change GitHub Actions to run on Ubuntu 18.04

.github/workflows/release.linux.yml

@@ -6,7 +6,7 @@ on:
 name: release (linux)
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     strategy:
       matrix:
         include:

.github/workflows/release.linux_static.yml

@@ -6,7 +6,7 @@ on:
 name: release (linux/static)
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     strategy:
       matrix:
         include:

.github/workflows/test.yml

@@ -2,7 +2,7 @@ on: push
 name: test
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/setup-go@v2
         with: