Trac 33624: fix doctest when sage scripts are not in SAGE_VENV/bin

We don't know `SAGE_VENV/bin` contains `sage`, but `sys.argv[0]` should
be the full path to `sage-ipython` and the same directory should also
have `sage`.

For a normal install, `os.path.dirname(sys.argv[0])` should be the same
as `SAGE_VENV/bin`.

For installs where the `sys.prefix` contains system python packages but
NOT the sagelib or the sage scripts, using `sys.argv[0]` should be ok
but using `SAGE_VENV/bin` fails this particular doctest.

This commit doesn't change any functionality, only a doctest.

src/sage/misc/sage_ostools.pyx

@@ -32,12 +32,11 @@ def have_program(program, path=None):
         True
         sage: have_program('there_is_not_a_program_with_this_name')
         False
-        sage: from sage.env import SAGE_VENV
-        sage: have_program('sage', os.path.join(SAGE_VENV, 'bin'))
+        sage: have_program('sage', os.path.dirname(sys.argv[0]))
         True
         sage: have_program('sage', '/there_is_not_a_path_with_this_name')
         False
-        sage: have_program('there_is_not_a_program_with_this_name', os.path.join(SAGE_VENV, 'bin'))
+        sage: have_program('there_is_not_a_program_with_this_name', "/bin")
         False
     """
     if path is None: