fix list-qt-official wrt types, search term.

[tsteven4]

Previously we accepted zero, one or more search terms for list-qt-official. However, the official installer ignores all terms after the first. This PR changes argument parsing for list-qt-official so only zero or one search terms are accepted.

This PR also fixes the mypy errors in installer.py:

aqt/installer.py:700:51: error: Incompatible types in assignment (expression
has type "list[str]", variable has type "str | None")  [assignment]
                commercial_search_args.search_terms = [rf"^.*{re.escape(version_str)}\.{re.escape(target_str)}.*$"]
                                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
aqt/installer.py:752:9: error: Incompatible types in assignment (expression has
type "str | None", variable has type "str")  [assignment]
            for version_str in (modules_ver, args.arch, args.archives[0] i...
            ^

The second was fixed by eliminating variable reuse of version_str by renaming, allowing different type signatures to be inferred by mypy for the different usages.

This resolves #965.