ArmVirtPkg: CI: use Python version from defaults template

Use the default Python version from the defaults template
(.azurepipelines/templates/defaults.yml) in the Windows and
Linux CI jobs.

Previous changes to the CI job templates make it necessary
to specify a version number, if Python shall be pulled
at CI runtime.

Signed-off-by: Oliver Steffen <osteffen@redhat.com>
Reviewed-by: Michael Kubacki <michael.kubacki@microsoft.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Chris Fernald <chfernal@microsoft.com>

ArmVirtPkg/PlatformCI/.azurepipelines/Ubuntu-GCC5.yml

@@ -15,6 +15,9 @@ pr:
   - master
   - stable/*
 
+variables:
+  - template: ../../../.azurepipelines/templates/defaults.yml
+
 jobs:
   - job: Platform_CI
     variables:
@@ -85,6 +88,7 @@ jobs:
         build_file: $(Build.File)
         build_flags: $(Build.Flags)
         run_flags: $(Run.Flags)
+        usePythonVersion: ${{ variables.default_python_version }}
         extra_install_step:
         - bash: sudo apt-get install qemu
           displayName: Install qemu