android_build_server_helper.py: Do not start server when stdout is a pipe

E.g. "autoninja > foo" would result in a file called "pipe:[394765110]"
being created.

Companion change: https://chromium-review.googlesource.com/c/chromium/src/+/6236987

Bug: None
Change-Id: Idbbee3eab136cb88f9658e033ac02bd90feee34d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/6278629
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Reviewed-by: Mohamed Heikal <mheikal@chromium.org>
Commit-Queue: Mohamed Heikal <mheikal@chromium.org>

android_build_server_helper.py

@@ -50,15 +50,18 @@ def _start_server(local_dev_server_path):
 
 def _set_tty_env():
     stdout_name = os.readlink('/proc/self/fd/1')
-    os.environ.setdefault("AUTONINJA_STDOUT_NAME", stdout_name)
+    # Anonymous pipes can't be opened. These look like "pipe:[394765110]".
+    ret = os.path.exists(stdout_name)
+    if ret:
+        os.environ.setdefault("AUTONINJA_STDOUT_NAME", stdout_name)
+    return ret
 
 
 @contextlib.contextmanager
 def build_server_context(build_id, use_android_build_server=False):
-    if not use_android_build_server:
+    if not use_android_build_server or not _set_tty_env():
         yield
         return
-    _set_tty_env()
     server_path = _get_server_path()
     _start_server(server_path)
     # Tell the build server about us.