Revert "buildroot: explicitly invoke runners via python3"

This reverts commit 33844711cd.

There are systems were our runners have no standard python3 location
available. They will fix the environment before invoking any further
utilities. Therefore, we cannot rely on `python3 foo.py` to work in our
ad-hoc containers.

This simply reverts the behavior back to using the shebang.

osbuild/buildroot.py

@@ -120,8 +120,6 @@ class BuildRoot(contextlib.AbstractContextManager):
             *[f"--bind-ro={b}" for b in nspawn_ro_binds],
             *[f"--bind={b}" for b in (binds or [])],
             *[f"--bind-ro={b}" for b in (readonly_binds or [])],
-            "--",
-            "python3",
             f"/run/osbuild/lib/runners/{self.runner}"
             ] + argv, check=check, **kwargs)