Trac 33624: don't assume SAGE_VENV/bin contains sage

Replace doctests in `have_program` that check in `SAGE_VENV/bin` by
similar ones in `/bin`, which should be more standard.

In any case `have_program` is scheduled for removal (see #32957).

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('sh', '/bin')
         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: