Build x86 on Windows with the built-in tools

[lgarron]

We build amd64 directly:

smimesign/.github/workflows/build-binaries.yml

Line 89 in e650daf

GOARCH=amd64 go build -o "build/amd64/smimesign.exe" -ldflags "-X main.versionString=${{ env.GIT_VERSION }}"

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:

smimesign/.github/workflows/build-binaries.yml

Lines 90 to 94 in e650daf

	- name: Switch MinGW to x86
	# Pinned hash from https://github.com/egor-tensin/setup-mingw/releases/tag/v2
	uses: egor-tensin/setup-mingw@f3c5d799aadf8fa230ac67a422b01dd085bbc96b
	with:
	platform: x86

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.