Always pull latest version

[jaxxstorm]

Allows setting the latest version to latest which will grab the latest version of tailscale. The default remains unchanged

Note: this updates the actual action script to find the latest version so we can validate the sha.

Fixes #28

[matanbaruch]

@jaxxstorm Can this be pushed? We are getting lots of bugs due to outdate version.

[andrewgbell]

Should add having actions on a much earlier version reduces the effectiveness of device posturing on linux.

[evilhamsterman]

When will this actually make it into a release? The latest release is from August 2023, there's been several commits to update the default version but there hasn't been an actual release so people following tailscale/github-action@v2 haven't received those updates. For that matter there's a bunch of other fixes and improvements in the last year and a half

[mpminardi]

@evilhamsterman we've just released a v3 of the action that has this change.

v3 is also going to start following the more common release pattern for GitHub actions where the v3 tag will be updated to refer to the latest v3.Y.Z release as new releases come out.