Use Ubuntu 20.04 for now

Signed-off-by: Yuki Okushi <jtitor@2k36.org>

Author: JohnTitor

.github/workflows/main.yml

@@ -15,11 +15,11 @@ jobs:
     strategy:
       matrix:
         include:
-          - os: ubuntu-latest
+          - os: ubuntu-20.04
             rust: stable
-          - os: ubuntu-latest
+          - os: ubuntu-20.04
             rust: beta
-          - os: ubuntu-latest
+          - os: ubuntu-20.04
             rust: nightly
           - os: macos-latest
             rust: stable
@@ -154,7 +154,7 @@ jobs:
 
   docker:
     name: Docker
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         target:
@@ -187,7 +187,7 @@ jobs:
 
   rustfmt:
     name: Rustfmt
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v1
       with:
@@ -198,7 +198,7 @@ jobs:
 
   build:
     name: Build Targets
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         target:
@@ -221,7 +221,7 @@ jobs:
 
   msrv:
     name: MSRV
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v1
       with:
@@ -232,7 +232,7 @@ jobs:
 
   miri:
     name: Miri
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v1
       with: