Bug 1869944 - Let mach use the mach site when building snaps. r=firefox-build-system-reviewers,sergesanspaille

The system environment under which the snaps are built doesn't always
match the tree that is built. When the versions of system python
packages doesn't match what is in the tree (like when upgrading
zstandard in the docker image, and building an older tree that wants
an older version), we get in a troublesome situation.

So just rely on the mach virtualenv.

Differential Revision: https://phabricator.services.mozilla.com/D197407

taskcluster/ci/snap-upstream-build/kind.yml

@@ -30,7 +30,6 @@ job-defaults:
         docker-image: {in-tree: snap-build-core22}
         max-run-time: 900
         env:
-            MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE: system
             UPLOAD_DIR: artifacts
             MOZ_STACKWALK_SYMBOLS_SERVER: '1'
     fetches: