Update the README.

Toolbox was renamed to Toolbx and has a new website.

README.md

-# Toolbox build
+# Toolbx build
 
 [![pipeline status](https://git.shore.co.il/shore/toolbox-build/badges/master/pipeline.svg)](https://git.shore.co.il/shore/toolbox-build/-/commits/master)
 
-Build [toolbox](https://github.com/containers/toolbox/).
+Build [toolbx](https://containertoolbx.org/).
 
 Until there's Debian packaging for it,
 [build it myself](https://git.shore.co.il/shore/toolbox-build/-/jobs/artifacts/master/raw/toolbox/builddir/src/toolbox?job=build).