Filter out "Downloading rust snapshot" percent indicator in mach's ou…

…tput based on sys.stdout.isatty() (Issue #5043).

python/servo/bootstrap_commands.py

@@ -38,7 +38,7 @@ def report(count, bsize, fsize):
         sys.stdout.flush()
 
     print("Downloading %s..." % desc)
-    dumb = os.environ.get("TERM") == "dumb"
+    dumb = (os.environ.get("TERM") == "dumb") or (not sys.stdout.isatty())
     PanickyUrlOpener().retrieve(src, dst, None if dumb else report)
     if not dumb:
         print()