fix(ci): install musl targets for preview build

[zemaj]

Summary

• pin the preview build workflow to Rust 1.90.0 (matching rust-toolchain.toml)
• install the musl cross targets alongside the matrix target so cargo can locate core

Fixes #355

Testing

• n/a (workflow change)

[zemaj]

Thanks for cutting this! Could we merge this fix soon? It updates preview-build to Rust 1.90.0 and adds the musl targets, which unblocks all the preview CI failures on #357 and #358. Once this lands we can re-run those PRs and confirm they go green.

[zemaj]

Looks like this toolchain PR was closed without merging, so the musl/darwin builds in #357/#358/#359/#362/#363 remain blocked (E0463). Could we reopen/merge this or spin a replacement so we can pick up the target installs?