ci: use Ubuntu 20.04

Let's use the latest LTS release.

Author: indygreg

.github/workflows/pyoxidizer.yml

@@ -13,7 +13,7 @@ jobs:
           # and the `Installing Rust` documentation when this changes.
           - '1.45.0'
         os:
-          - 'ubuntu-18.04'
+          - 'ubuntu-20.04'
           - 'macos-10.15'
           - 'windows-2019'
     continue-on-error: true
@@ -22,7 +22,7 @@ jobs:
       IN_CI: '1'
     steps:
       - name: Install Linux system packages
-        if: ${{ matrix.os == 'ubuntu-18.04' }}
+        if: ${{ matrix.os == 'ubuntu-20.04' }}
         run: |
           sudo apt-get install -y libyaml-dev snapcraft