[build] Specify source_root

See more details about source_root in https://fxrev.dev/515901.

Change-Id: I56ebb25f84a0c1595f548f34548b66b207ecd7c5
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/pyyaml/+/515687
Reviewed-by: Shai Barack <shayba@google.com>