#36709 guix: compute --build triplet when lowering, not at load time

Converts `building-on` in `contrib/guix/toolchains.scm` from an eagerly evaluated variable into a procedure evaluated when lowering derivations. This ensures cross-toolchain packages use the target system passed via Guix's `--system` flag rather than the host system running `guix shell`.

Problem: When running Guix with `--system` under emulation on a host of a different architecture (for example, `--system=aarch64-linux` on an x86_64 host), `guix shell` loads manifests before applying `--system`. As a result, `building-on` captures the host's `%current-system`, causing cross toolchains like `gcc-cross-sans-libc` to configure with mismatched `--build` triplets and fail to build.

Evidence of importance:

• PR description demonstrates build failure: 'checking for CET support... configure: error: compiler and assembler with CET support are required for --enable-cet' when building under `--system=aarch64-linux` on x86_64
• PR description confirms native builds produce identical toolchain derivations to master on both `linux_static.scm` and `manifest_gui.scm`

Risk of a mistake: medium (build). A mistake in Guix toolchain definitions could alter compiler build flags or break reproducible release builds across supported architectures.

Changed since the previous assessment: No functional changes; the previous description was generated before the patch was available.

Ready

Nothing known that would invalidate a review now.

Waiting on (from the thread read):

• reviewer: further review after author answered sedited's question 2026-10-30
• reviewer: code review following author reply and Concept ACK 2026-10-30

Positive

support without stated reasons, no open objection (hebasto)

Thread notes: Initial record. hebasto offered a Concept ACK, and sedited asked about the author's motivation and setup, which jespada answered. / Initial record. sedited asked why the author ran into this, which was answered; hebasto left a Concept ACK.

Objections, suggestions, and questions:

Support:

• hebasto (member) (Concept ACK, 2026-10-30): Concept ACK. [not substantive]

Participants:

• hebasto (member, support, 1 statements 2026-10-30 to 2026-10-30): Concept ACK
• sedited (member, question, 1 statements 2026-10-30 to 2026-10-30): asked how and why the author ran into the issue

8 lines under test/bench/ci.

• contrib/guix/toolchains.scm +4/-4

The short text the ranking pass compares across PRs.

Converts the `building-on` triplet resolution in `contrib/guix/toolchains.scm` from an eagerly evaluated variable into a procedure evaluated when lowering derivations. When running Guix with `--system` on a foreign host via emulation (such as building for aarch64 on x86_64), manifests previously evaluated `%current-system` at module load time, generating an incorrect `--build` triplet that broke cross-toolchain configuration for `gcc-cross-sans-libc`. The change fixes emulated cross-system Guix builds without modifying the derivations produced for native release builds.

P3 · platform fix (ranking pass: Moved up from 43: fixes an actual Guix build failure under foreign-architecture emulation)

Fixes a Guix build failure when running under foreign architecture emulation with `--system`. Guix shell currently captures the host `%current-system` at load time before applying `--system`, causing cross toolchains like `gcc-cross-sans-libc` to fail configuration due to mismatched triplets.

Membership: Matches 'Build system' label and edits 'contrib/guix/toolchains.scm', which configures Guix cross-toolchain packages.

Factors: security/stability 0, bug 1, performance 0, user value 0, leverage 1