gulp skillmap in the main pxt directory will build the skillmap. This is part of the default gulp command, and runs with our build, but you can also run it separately; the command builds the skillmap and copies the generated index.html file into webap/public as skillmap.html, as well as coping all the built JS and CSS files into built/web/skillmap.
Run pxt skillmap --serve in the target directory to build and serve the skillmap locally. This command copies the appropriate built files (pxtlib.js, target.js, semantic.css) into your local skillmap directory to serve, so you may also need to run the appropriate build commands to generate those files.
Use the ?nolocalhost=1 flag to load the default content for the skillmap.