nightly: fix arm64 cross-link (musl cross toolchain)

[thepagent]

Follow-up to #21 / #22.

Problem

The first real multi-arch nightly run (33774193328) failed the arm64 legs at link time (grok, the amd64-only variant, passed — which localized it to the arm64 cross-compile):

error: linking with `cc` failed
  ... aarch64-unknown-linux-musl/.../crt1.o ...
collect2: error: ld returned 1 exit status

Root cause: musl-tools only provides the build host's musl-gcc (x86_64-linux-musl-gcc). Cross-linking a static aarch64-unknown-linux-musl binary needs an aarch64 musl linker, which it does not ship — so the link dies at crt1.o.

My earlier verification for #22 had a blind spot: I built the arm64 image natively on an M4, which is not a cross-compile, so it never exercised the cross-linker path CI uses.

Why not cargo-zigbuild

Tried it (zig 0.13 + zigbuild 0.19.8, and zig 0.14 + zigbuild 0.20.1). Both reject rustc's aarch64 linker arg:

error: unsupported linker arg: --fix-cortex-a53-843419

Fix

Install the official musl.cc cross toolchain for the target arch and point the Rust target's linker/CC at its <triple>-gcc. Strip with the toolchain's <triple>-strip (host binutils strip cannot read a foreign-arch ELF — "Unable to recognise the format"). The amd64 leg is kept identical in shape (its own musl cross toolchain instead of musl-tools), so both arches build the same way.

Verification

Built the whole image for linux/amd64,linux/arm64 from an amd64 builder context on an M4 (the real CI cross-compile path this time, not a native build):

manifest list: 2 platform manifests
amd64 leg: uname x86_64,  openab-pty & kiro-cli ELF e_machine 0x3E (x86-64)
arm64 leg: uname aarch64, openab-pty & kiro-cli ELF e_machine 0xB7 (AArch64)

Both legs compile, cross-link, strip, install the correct-arch kiro CLI, and produce a valid two-platform manifest.

Note

Depends on musl.cc being reachable at build time (retried 5x). If that host's flakiness becomes a problem, a follow-up can vendor the toolchains or switch to a mirror.

[thepagent]

Superseding this with native arm64 runners. GitHub provides free ubuntu-24.04-arm hosted runners for public repos, so the cleaner fix is to build each arch on its own native runner (no cross toolchain, no QEMU, no musl.cc dependency) and merge into a manifest — which also lets the arm64 leg be smoke-tested for real. New PR to follow.