dev: Ensure HOST and PORT are stable when generating command doc

These are used in `nextstrain view --help` output, and setting them
during development (e.g. for `make -C doc livehtml`) would otherwise
influence the generated doc.

Author: tsibley

devel/generate-command-doc

@@ -46,6 +46,10 @@ os.environ.update({
 
     # Ensure we detect a browser for stable `nextstrain view` output.
     "BROWSER": "/bin/true",
+
+    # Ensure HOST and PORT are stable for `nextstrain view` output.
+    "HOST": "127.0.0.1",
+    "PORT": "4000",
 })
 
 from nextstrain.cli import make_parser