20846: Use Python build variables more consistently

20846: Use Python build variables more consistently

Rather than have the virtualenv process use its own python command,
ensure PYTHON3_EXECUTABLE points to a specific minor version, and use it
throughout the entire build process.

The changes to the virtualenv manipulation with sed are meant to clean
things up with fewer commands and tighter regexps, but not make any
functional change beyond referring to a specific minor Python version.

Arvados-DCO-1.1-Signed-off-by: Brett Smith <brett.smith@curii.com>