bazel: fix python stub paths.

Since the 0.21 upgrade, the host `$PATH` is not forwarded anymore by
default to the sandboxes in charge to realize Bazel actions. This
default change broke the `py_binary` rule among other things.

Every python binary is wrapped in a stub in charge to setup the
execution environment. Currently, this stub's shebang points to a
`/usr/bin/env python` which cannot be resolved with the current
`$PATH`.
This results in breaking any build pipeline requiring the use of
python at some point. On top of the incorrect shebang, the stub
template is unable to find the actual python binary using
`SearchPath`.

This PR fixes those two things by re-writing the stub template shebang
to the actual python binary and by substituting the faulty default
python binary lookup to the right one.