Prepend third_party/pyyaml to python sys.path (#54023)

Previously we were appending Flutter's fork of third_party/pyyaml to the Python sys.path. In the case where a bot image or local install already has pyyaml installed elsewhere on the path, ours will fail to be picked up. Instead prepend to the path.

Issue spotted by jsimmons@.

Related: #54001

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

tools/fuchsia/dart/gen_dart_package_config.py

@@ -15,7 +15,7 @@
 import sys
 
 THIS_DIR = os.path.abspath(os.path.dirname(__file__))
-sys.path += [os.path.join(THIS_DIR, '..', '..', '..', 'third_party', 'pyyaml', 'lib')]
+sys.path.insert(0, os.path.join(THIS_DIR, '..', '..', '..', 'third_party', 'pyyaml', 'lib'))
 import yaml
 
 DEFAULT_LANGUAGE_VERSION = '2.8'

tools/pub_get_offline.py

@@ -15,7 +15,7 @@
 import sys
 
 THIS_DIR = os.path.abspath(os.path.dirname(__file__))
-sys.path += [os.path.join(THIS_DIR, '..', 'third_party', 'pyyaml', 'lib')]
+sys.path.insert(0, os.path.join(THIS_DIR, '..', 'third_party', 'pyyaml', 'lib'))
 import yaml  # pylint: disable=import-error, wrong-import-position
 
 SRC_ROOT = os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))