Revert "Use ubuntu-20.04 instead of ubuntu-latest"

This reverts commit f170b1f.

.github/workflows/clippy.yml

@@ -30,7 +30,7 @@ env:
 jobs:
   base:
     # NOTE: If you modify this job, make sure you copy the changes to clippy_bors.yml
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
 
     steps:
     # Setup

.github/workflows/clippy_bors.yml

@@ -19,7 +19,7 @@ defaults:
 
 jobs:
   changelog:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
 
     steps:
     - uses: rust-lang/simpleinfra/github-actions/cancel-outdated-builds@master
@@ -53,12 +53,12 @@ jobs:
     needs: changelog
     strategy:
       matrix:
-        os: [ubuntu-20.04, windows-latest, macos-latest]
+        os: [ubuntu-latest, windows-latest, macos-latest]
         host: [x86_64-unknown-linux-gnu, i686-unknown-linux-gnu, x86_64-apple-darwin, x86_64-pc-windows-msvc]
         exclude:
-        - os: ubuntu-20.04
+        - os: ubuntu-latest
           host: x86_64-apple-darwin
-        - os: ubuntu-20.04
+        - os: ubuntu-latest
           host: x86_64-pc-windows-msvc
         - os: macos-latest
           host: x86_64-unknown-linux-gnu
@@ -147,7 +147,7 @@ jobs:
 
   metadata_collection:
     needs: changelog
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
 
     steps:
      # Setup
@@ -166,7 +166,7 @@ jobs:
 
   integration_build:
     needs: changelog
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
 
     steps:
     # Setup
@@ -224,7 +224,7 @@ jobs:
         - 'rust-lang-nursery/failure'
         - 'rust-lang/log'
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
 
     steps:
     # Setup
@@ -265,7 +265,7 @@ jobs:
   end-success:
     name: bors test finished
     if: github.event.pusher.name == 'bors' && success()
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     needs: [changelog, base, metadata_collection, integration_build, integration]
 
     steps:
@@ -275,7 +275,7 @@ jobs:
   end-failure:
     name: bors test finished
     if: github.event.pusher.name == 'bors' && (failure() || cancelled())
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     needs: [changelog, base, metadata_collection, integration_build, integration]
 
     steps: