build: ship static musl release binaries instead of glibc-linked ones

[fcostaoliveira]

Fixes #1.

The published -gnu artifacts are built on ubuntu-24.04 and require GLIBC_2.39, so they cannot start on most current LTS targets:

Distro	glibc	Before	After
Ubuntu 24.04	2.39	✅	✅
Ubuntu 22.04	2.35	❌	✅
Debian 12	2.36	❌	✅
RHEL 9	2.34	❌	✅

Why it went unnoticed

The failure is quiet. The binary installs, command -v finds it, and only the loader rejects it — so --version returns a linker error rather than a version string. A harness that verifies the installed build by reading --version can't distinguish that from "wrong version", reports the tool unavailable, and the benchmark skips while the run stays green.

That's exactly what happened: five benchmark suites skipped on a jammy host, CI green, nothing measured.

The existing verification step runs the binary on the build host — the one machine where it's guaranteed to work. A passing smoke test there says nothing about older hosts, which is how this shipped.

The change

Both targets move to *-unknown-linux-musl. Static linking removes the glibc floor entirely, which for a benchmark binary copied onto whatever host is under test is the property that actually matters — the target distro is the one variable you don't control.

Viable here because TLS is rustls + webpki-roots, not OpenSSL: no C TLS dependency, and no reliance on the host's ca-certificates either.

Also adds an assertion that the artifact carries no GLIBC_* symbol versions, failing the build if it does — so this can't silently regress.

Verified locally

Built resque-bench for x86_64-unknown-linux-musl from this branch's config:

EXIT=0
file: ELF 64-bit LSB pie executable, x86-64, static-pie linked
max GLIBC symbol: NONE
runs: resque-bench 0.1.0

Two notes for review

1. Asset names change *-gnu → *-musl. v0.1.0 is days old so I doubt anything pins the old names, but it is a rename.
2. musl-tools may be unnecessary. My local build succeeded without musl-gcc. I've kept the install step because ring (pulled in by rustls) compiles C and asm and can need it under other toolchain configurations — but if you'd rather not pay the apt-get, it can likely go.