test(e2e): restore musl DNS probe coverage for guest prebuilt artifacts #3009

Description

The rootless Podman guest prebuilt-artifact E2E mode currently skips the musl DNS probe case in transparent_tcp because the test builds e2e/support/musl-dns-probe.c at runtime, but e2e/run.sh --tests-in-vm copies only compiled Rust test artifacts into the VM.

Context

PR #3004 moves the rootless Podman E2E tests into the Nix-managed Ubuntu 26.04 guest so dynamic fixtures and sandboxes share the same VM network context. That exposed a host-source assumption in tests/transparent_tcp.rs: rootless_podman_musl_getaddrinfo_uses_udp_policy_dns locates ../support/musl-dns-probe.c through CARGO_MANIFEST_DIR and invokes mise x -- zig cc at test runtime.

In guest prebuilt-artifact mode, the source tree and mise toolchain are intentionally not present in the VM. Embedding the C source in the Rust test would make the test pass, but that keeps runtime compilation inside the test and is not the preferred shape for this lane.

Definition of Done

• The musl DNS probe is built or packaged as an explicit E2E artifact before the guest test run.
• e2e/run.sh --tests-in-vm copies the probe artifact into the Nix guest when transparent_tcp is selected.
• transparent_tcp uses the supplied probe artifact in guest prebuilt-artifact mode instead of compiling C source at runtime.
• rootless_podman_musl_getaddrinfo_uses_udp_policy_dns is re-enabled for guest prebuilt-artifact runs.
• The full Ubuntu 26.04 rootless Podman e2e_podman lane passes with the restored probe coverage.