CI: fix Alpine static builds and ship a real 32-bit wibo-i686

[marijnvdwerf]

Two independent fixes to the release/CI pipeline. Both are reproducible on a fresh checkout of main.

1. Alpine static builds are broken (all four static jobs fail)

alpine:latest advanced to 3.24.x (clang 22 / gcc 15.2 / cmake 4.2) and no longer pulls in gcc transitively. The static toolchain links with clang --target=...-musl -static -fuse-ld=lld, which needs the GCC runtime (crtbeginT.o, libgcc, libgcc_eh, crtend.o) plus the musl crt/libc. Without them the CMake compiler check fails:

ld.lld: error: cannot open crtbeginT.o: No such file or directory
ld.lld: error: unable to find library -lgcc
ld.lld: error: unable to find library -lgcc_eh
ld.lld: error: cannot open crtend.o: No such file or directory

Fix: install gcc and musl-dev explicitly in Dockerfile.

2. wibo-i686 is actually a 64-bit binary

The Export binary step omits platforms:, so it always rebuilds for the host (linux/amd64) regardless of the matrix entry. The matrix platform only applies to the test step, not the exported artifact. As a result the static 32-bit release assets (wibo-i686, wibo-i686-debug) have been 64-bit x86-64 ELF binaries — even though the README advertises wibo-i686 as the primary 32-bit download.

This has been the case for every release since the i686/x86_64 split was introduced at 1.0.0-beta.3:

Release	wibo-i686 arch
1.0.0-beta.2 (single wibo asset)	ELF 32-bit ✅
1.0.0-beta.3 … 1.0.3	ELF 64-bit ❌
1.1.0 / 1.1.1	ELF 64-bit ❌

Fix: pass the matrix platform to the export build so the exported binary matches the platform that was tested.

Validation

Local Docker builds mirroring CI:

• linux/amd64, --target build + ctest: builds and passes.
• linux/386, --target export: produces a flat dist/wibo that is now ELF 32-bit LSB executable, Intel 80386.

The two changes are independent and can be split into separate PRs if preferred.