bootstrap: Invoke "pip" through Python.
bootstrap: Invoke "pip" through Python.

Invoking "pip" as a command-line executable falls into the problem of
shebang length truncation if checked out into a too-long subdirectory.
Invoke "pip" through the Python binary.

BUG=None
TEST=None

Review-Url: https://codereview.chromium.org/2816633002

luci/recipes-py,luci/recipes-py