Update github actions to latest versions

Allow 1.x versions of swift-argument-parser

Resolve to current version 1.1.2.

Allow workflow to be manually triggered

Use human-readable flags in README.md

Most users probably want localized date formats, use these in examples
in the project README even if it's not the actual default for the tool.

Add LICENSE.txt to Xcode project

Install LICENSE.txt along with vivtool.

Propagate error message from libviv