The initial intent was to use an interpreter named `python_for_test` and not
`python`, so that software do not accidentally pick up this python during
compilation.
6033e4fa (software/slapos-sr-testing: fix python3 profile, 2020-12-01)
was a quick fix for some python3 compatibility issues discovered after merge,
but it was not correct, because by using ${python-interpreter:eggs}, it was
installing the default python-interpreter section, which has an interpreter
named `python`.

This caused issues while building proxysql, which assumes /usr/bin/env python
is python2.

The fix is to override python-interpreter directly, we don't need another
section eggs here.