Run gyp_uv with CFG_PYTHON instead of directly

The syntax of the script requires python < 3, and so does our build system so we
can just use CFG_PYTHON to run the script. This prevents build failures where
`python` is actually python3 or later.
auto merge of #8821 : alexcrichton/rust/fix-libuv-build, r=huonw

The syntax of the script requires python < 3, and so does our build system so we
can just use CFG_PYTHON to run the script. This prevents build failures where
`python` is actually python3 or later.