[Bug]: Flaggie requires privileged access to run the --pretend option

[Kangie]

kangie@monolith ~ $ flaggie --version
flaggie 0.99.8

What I did:

kangie@monolith ~ $ flaggie --pretend '*/*' '+use::potato' --no-package-manager

What I got:

Traceback (most recent call last):
  File "/usr/lib/python-exec/python3.11/flaggie", line 8, in <module>
    sys.exit(entry_point())
             ^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/flaggie/__main__.py", line 367, in entry_point
    sys.exit(main(*sys.argv))
             ^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/site-packages/flaggie/__main__.py", line 347, in main
    save_config_files(config_files, confirm_cb=confirm_cb)
  File "/usr/lib/python3.11/site-packages/flaggie/config.py", line 193, in save_config_files
    with tempfile.NamedTemporaryFile(mode="w",
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/tempfile.py", line 563, in NamedTemporaryFile
    file = _io.open(dir, mode, buffering=buffering,
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/tempfile.py", line 560, in opener
    fd, name = _mkstemp_inner(dir, prefix, suffix, flags, output_type)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.11/tempfile.py", line 256, in _mkstemp_inner
    fd = _os.open(file, flags, 0o600)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
PermissionError: [Errno 13] Permission denied: '/etc/portage/package.use/tmplx_mj8wo'

What I expected

kangie@monolith ~ $ flaggie --pretend '*/*' '+use::potato' --no-package-manager
diff --git a/tmp/package.use/wayland b/tmp/package.use/tmpaijlpnc2
index c603ae2..6a2dfb5 100644
--- a/tmp/package.use/wayland
+++ b/tmp/package.use/tmpaijlpnc2
@@ -1,2 +1,2 @@
*/* {+potato+} wayland
media-libs/libsdl2 gles2
Time: 0h:00m:03s 

Can we just write out to /tmp instead? I don't always want to invoke Flaggie from a user that can actually write to /etc/portage/package.use