(maint) Remove PT source vars from pr_testing env

These were included to help when calling tasks/install_linux.sh
directly, but the script itself has these defaults, and they should not
need to be passed in now.

Author: jpartlow

.github/workflows/pr_testing.yaml

@@ -13,8 +13,6 @@ env:
   # Environment variables needed when running the task install scripts
   # manually.
   PT__installdir: ${{ github.workspace }}
-  PT_apt_source: "https://apt.voxpupuli.org"
-  PT_yum_source: "https://yum.voxpupuli.org"
 
 jobs:
   shellcheck: