build: make sure cwd is in Python's sys.path

Apparently some packaging systems (homebrew does this for example) change
`sys.path` before they run any Python script to ensure that only the correct
Python modules can be loadable.

We need to make sure cwd is in `sys.path` since we need to load
`wscript_build.py` from there.
README: mention build result being located at build/mpv