ci: build the WASM modules in parallel jobs

The website workflow built all four WASM modules (uutils, grep, find,
diffutils) serially in a single job. Split them so they compile in
parallel: coreutils keeps its own job (it needs the l10n copy, locales.js
and feat_wasm command parsing), while grep/find/diffutils share one matrix
job (grep/find pull in onig_sys and need a WASI sysroot; diffutils is pure
Rust). Each leg uploads its .wasm plus numbered command/version fragments
that the build job stitches into commands.js / version.js; SITE_VERSION is
generated there since it describes the site repo. Add Swatinem/rust-cache
to every build to speed up warm runs.

Author: sylvestre