Use virtualenv Python and "disable" debugging

Replace both the standard and debugging Python interpreters with the
standard interpreter from our virtualenv. This is correct for the
former, and for the latter, should suffice to make the build system
happy, while not ever actually being used during a wheel build (so it
won't matter that it isn't a debug interpreter).

This should be sufficient that Drake can build. However, due to some
nasty business with libcrypt, which for some reason is not used by
Python 3.6, but is required by Python 3.7, we are not yet able to
produce a PyPI-usable wheel.

See also pypa/manylinux#305 for an explanation
of why this is broken.

fixup

image/pip-drake.patch

@@ -195,3 +195,23 @@ index d98b81c59..fb9bcd86c 100644
 
    ExecuteExtraPythonCode(m);
  }
+diff --git a/tools/py_toolchain/interpreter_paths.bzl b/tools/py_toolchain/interpreter_paths.bzl
+index 1608e49b1..d6b5d5e07 100644
+--- a/tools/py_toolchain/interpreter_paths.bzl
++++ b/tools/py_toolchain/interpreter_paths.bzl
+@@ -2,13 +2,13 @@
+
+ # Default value of interpreter_path used by the py_runtime in the default
+ # Python toolchain registered on the @platforms//os:linux platform.
+-LINUX_INTERPRETER_PATH = "/usr/bin/python3"
++LINUX_INTERPRETER_PATH = "/usr/local/bin/python"
+
+ # Default value of interpreter_path used by the py_runtime in the Python debug
+ # toolchain registered on the @platforms//os:linux platform when the
+ # --extra_toolchains=//tools/py_toolchain:linux_dbg_toolchain command line
+ # option is given.
+-LINUX_DBG_INTERPRETER_PATH = "/usr/bin/python3-dbg"
++LINUX_DBG_INTERPRETER_PATH = "/usr/local/bin/python"
+
+ # Default value of interpreter_path used by the py_runtime in the default
+ # Python toolchain registered on the @platforms//os:osx platform.