Build `x86` on Windows with the built-in tools

We build `amd64` directly: https://github.com/github/smimesign/blob/e650daf6eaadf85c763fa06dcfb0e4d794d293dc/.github/workflows/build-binaries.yml#L89

But I had trouble figuring out how to build for `GOARCH=386` without errors. Right now, we use https://github.com/egor-tensin/setup-mingw to switch to 32-bit builds, which pulls down a bunch of tooling and takes a few minutes:

https://github.com/github/smimesign/blob/e650daf6eaadf85c763fa06dcfb0e4d794d293dc/.github/workflows/build-binaries.yml#L90-L94

I can't help but feel there must still be a nice (or at least much faster) way to do this using the built-in tools from Actions.

Inizia con .github/workflows/build-binaries.yml intorno alle righe 89-94, confrontando la build amd64 con il percorso GOARCH=386 e il passaggio setup-mingw. Esegui il workflow Windows Actions mentre testi l’approccio basato sugli strumenti integrati; il lavoro è concluso quando i binari x86 vengono compilati correttamente senza setup-mingw e il workflow rimane funzionante.

Scritto dal modello di indicizzazione a partire dal testo della issue.