EmulatorPkg: CI: Use Fedora 35 container (Linux only)

Run the Linux jobs of the EmulatorPkg platform CI inside a container,
in the same way the general CI does now. Make use of the default image
specified in the defaults.yml template.

Use Python from the container image, do not download at 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>

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

@@ -79,6 +79,8 @@ jobs:
     pool:
       vmImage: $(vm_image)
 
+    container: ${{ variables.default_linux_image }}
+
     steps:
     - template: ../../../.azurepipelines/templates/platform-build-run-steps.yml
       parameters:
@@ -89,4 +91,4 @@ jobs:
         build_file: $(Build.File)
         build_flags: $(Build.Flags)
         run_flags: $(Run.Flags)
-        usePythonVersion: ${{ variables.default_python_version }}
+        usePythonVersion: '' # use Python from the container image