extensions-tool: Fix security warnings about format string injection

extensions-tool: Fix security warnings about format string injection

Fixes:
```
../subprojects/extensions-tool/src/command-upload.c: In function ‘response_get_token’:
../subprojects/extensions-tool/src/command-upload.c:126:7: warning: format not a string literal and no format arguments [-Wformat-security]
  126 |       g_set_error (error, G_IO_ERROR, G_IO_ERROR_FAILED, msg);
      |       ^~~~~~~~~~~
../subprojects/extensions-tool/src/command-upload.c: In function ‘session_get_login_token’:
../subprojects/extensions-tool/src/command-upload.c:294:20: warning: format not a string literal and no format arguments [-Wformat-security]
  294 |                    detail ? detail : soup_status_get_phrase (status));
      |                    ^~~~~~
../subprojects/extensions-tool/src/command-upload.c: In function ‘session_upload_file’:
../subprojects/extensions-tool/src/command-upload.c:357:20: warning: format not a string literal and no format arguments [-Wformat-security]
  357 |                    detail ? detail : soup_status_get_phrase (status));
      |                    ^~~~~~
```

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3764>