Test CI ubuntu 20.04 early.

Author: jacobly0

.github/workflows/cmake.yml

@@ -10,7 +10,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        runs-on: [ubuntu-latest, macos-latest, windows-latest]
+        runs-on: [ubuntu-20.04, macos-latest, windows-latest]
         build-type: [Debug, Release]
     if: always()
     runs-on: ${{matrix.runs-on}}
@@ -120,7 +120,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        runs-on: [ubuntu-latest, macos-latest, windows-latest]
+        runs-on: [ubuntu-20.04, macos-latest, windows-latest]
         build-type: [Debug, Release]
     if: always()
     runs-on: ${{matrix.runs-on}}
@@ -245,7 +245,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        runs-on: [ubuntu-latest, macos-latest, windows-latest]
+        runs-on: [ubuntu-20.04, macos-latest, windows-latest]
         build-type: [Debug]
     if: always()
     runs-on: ${{matrix.runs-on}}
@@ -287,7 +287,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        runs-on: [ubuntu-latest, macos-latest, windows-latest]
+        runs-on: [ubuntu-20.04, macos-latest, windows-latest]
         build-type: [Debug]
     if: always()
     runs-on: ${{matrix.runs-on}}