ci: expose isolated toolchain to broker builder (#1633)

* fix(ci): expose isolated Rust toolchain to broker builder

* fix(ci): self-validate broker producer workflow

* fix(ci): build broker on executable filesystem

* fix(ci): stage broker source for isolated builder

* docs(trail): record broker producer repair

* docs(trail): complete broker repair record

* fix(ci): anchor isolated broker build cwd

* test(ci): pin isolated cargo path

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Co-authored-by: Proactive Runtime Bot <agent@agent-relay.com>