Minify assets/pixelex.js into priv/static/pixelex.js and write its SRI hash.
mix pixelex.buildOnly maintainers need this; the built file is committed and ships in the
package. mix test fails if the two drift apart, so a change to the source
that forgets this step cannot be released.
Needs esbuild on PATH, or ESBUILD pointing at one. Phoenix apps already
have a copy at ~/.cache/phx-esbuild/package/bin/esbuild.