Bug 1759084: Don't redundantly activate site in upload_symbols.py r=ahal

`upload_symbols.py` is always executed within a Mach site (otherwise
activating the `common` command site would fail), therefore
re-activating `common` is redundant.

Note that `zstandard` is available from the system python, but only
some upload-symbols jobs were using it - the "use system python"
inference was not finding `zstandard` on Linux workers because they
didn't have `pip` installed. So, I've explicitly set the "native
packages source" to "SYSTEM" to force them to find and use it.

Differential Revision: https://phabricator.services.mozilla.com/D140982

taskcluster/ci/system-symbols/kind.yml

@@ -22,6 +22,9 @@ job-defaults:
         kind: other
         tier: 2
     worker-type: b-linux
+    worker:
+        env:
+            MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE: system
 
 jobs:
     mac:

taskcluster/ci/upload-symbols/kind.yml

@@ -38,6 +38,7 @@ job-template:
             # {level} gets replaced in the upload_symbols transform
             SYMBOL_SECRET: "project/releng/gecko/build/level-{level}/gecko-symbol-upload"
             MAKECAB: /builds/worker/fetches/makecab/makecab
+            MACH_BUILD_PYTHON_NATIVE_PACKAGE_SOURCE: system
     run:
         using: mach
         mach: {artifact-reference: "python toolkit/crashreporter/tools/upload_symbols.py <build/public/build/target.crashreporter-symbols-full.tar.zst>"}

toolkit/crashreporter/tools/upload_symbols.py

@@ -23,7 +23,6 @@ import redo
 import requests
 import shutil
 import sys
-from mozbuild.base import MozbuildObject
 
 log = logging.getLogger("upload-symbols")
 log.setLevel(logging.INFO)
@@ -57,9 +56,6 @@ def get_taskcluster_secret(secret_name):
 
 
 def main():
-    config = MozbuildObject.from_environment()
-    config.activate_virtualenv()
-
     logging.basicConfig()
     parser = argparse.ArgumentParser(
         description="Upload symbols in ZIP using token from Taskcluster secrets service."
@@ -90,8 +86,6 @@ def main():
         import gzip
         import tarfile
         import tempfile
-
-        config._ensure_zstd()
         import zstandard
 
         def prepare_zip_from(archive, tmpdir):