build: don't pass python override to V8 build

If the `configure.py` script is executed by a Python binary that is
not the one on the PATH it will create a `python` symlink in
`out/tools/bin` and prefix that to the PATH so it is used instead of
the one that otherwise would have been found on the PATH. This is
done so that gyp scripts shelling out to `python` execute with the
same version of Python as used to run the configure script.

V8's build uses V8's build toolchain (i.e. not gyp) and currently that
is incompatible with Python 3. Prevent prefixing the PATH for the V8
build so that it picks up `python` from the unprefixed PATH. This will
allow us to build Node.js with Python 3 but still use Python 2 to build
V8 in the CI.

PR-URL: https://github.com/nodejs/node/pull/38969


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>