Update Ubuntu builds to 18.04

Ubuntu 16.04 is EOL and breaks for building.

.github/workflows/build-mudlet.yml

@@ -17,7 +17,7 @@ jobs:
       matrix:
         include:
           # oldest OS supported for maximum compatibility of built AppImage
-          - os: ubuntu-16.04
+          - os: ubuntu-18.04
             buildname: 'ubuntu / gcc'
             triplet: x64-linux
             compiler: gcc_64
@@ -145,7 +145,7 @@ jobs:
       if: runner.os == 'Linux'
       run: |
         # workaround to install libzip-dev in Github Actions
-        sudo apt-add-repository "ppa:ondrej/php" -y
+        #sudo apt-add-repository "ppa:ondrej/php" -y
         sudo apt-get update
 
         # Install from vcpkg everything we can for cross-platform consistency