v0.0.6: Add fallback cross install if there is no verison specified (#10)

This adds a fallback install for cross if the cross version was not set.

v0.0.5

Adds new valid values to the cross-version field:

• "main" to pull from https://github.com/cross-rs/cross
• "git:XYZ" to pull from the git hash of XYZ.

v0.0.4

Make some input handling a bit cleaner internally.

v0.0.3

Update to handle the changes of https://github.blog/changelog/2022-10-11-github-actions-deprecating-save-state-and-set-output-commands/

v0.0.2

Add some comments to the steps so it's easier to discern what's going on.

v0.0.1 release

Update README.md