fix(wp2static): Downloads WP2Static plugins from repo releases

[nvnivs]

Resolves #50

• Downloads wp2static from repo releases
• Downloads s3-addon from repo releases
• Adds variables to control the versions of the plugins

[petewilcock]

Looks really promising @z0c! Seems Github is having a blip with the tests, but I'll run the branch locally too for a test. Many thanks for the improvement! 👍

[nvnivs]

thanks @petewilcock.

I suspect that some of the issues that actions is having is due to this being a branch of a branch on a fork, at least these type of errors:

error: pathspec 'wp2static_fix' did not match any file(s) known to git

[nvnivs]

Also one thing i noticed, recently https://github.com/leonstafford/wp2static was migrated to https://github.com/WP2Static/wp2static

Not sure if the same is planned for https://github.com/leonstafford/wp2static-addon-s3, if it does the repo redirection should work on the curl command and get the archive downloaded.