[testnet] CI: pin cargo-rdme to 1.5.0 to fix readme lint on the pinned toolchain

[ndr-ds]

Motivation

The lint-check-for-outdated-readme job is now failing deterministically on every testnet_conway PR. cargo install cargo-rdme --locked resolves to the freshly released cargo-rdme 1.5.1, whose lockfile pulls zmij 1.0.21, which uses hint::select_unpredictable — stabilized in Rust 1.88. This branch's pinned toolchains (stable 1.86.0, nightly-2025-04-03) cannot build it, so the install step exits 101 before the README check even runs. The same job passed on this branch as recently as 2026-06-11 (PRs #6474, #6476) and broke on 2026-06-12 (#6488), bracketing the upstream release. main is unaffected (toolchain 1.95).

Proposal

Pin the tool: cargo install cargo-rdme --locked --version 1.5.0, with a comment explaining why. cargo-rdme 1.5.0 declares rust-version = 1.86.0.

Test Plan

Verified locally that cargo install cargo-rdme --locked --version 1.5.0 builds successfully on rustc 1.86.0 (this branch's stable pin) and the binary runs. This PR's own lint-check-for-outdated-readme job exercises the pinned install directly, so its CI result is the end-to-end proof.

Release Plan

• CI-only change on testnet_conway; nothing to deploy. main does not need this (newer toolchain), though the pin could be ported there for reproducibility if desired.

[ndr-ds]

Merging to make CI green again