fix(ci): install Rust via rustup on the self-hosted ADF lane

[AlexMikhalev]

Problem

The nightly ADF Agent Validation workflow failed on every scheduled run since 4 October (9 consecutive failures, latest 37907419012). Two independent faults, the second hidden behind the first.

1. Unresolvable action

The job died during "Prepare all required actions", before any Rust step ran:

##[error]Unable to resolve action dtolnay/rust-action, repository not found

dtolnay/rust-action does not exist. Install through rustup instead, matching the pattern release-lsp.yml already uses on its self-hosted lane. Pinned to 1.96 to follow the repo root rust-toolchain.toml (fleet standard, ADR-0006); stable would drift from that pin.

2. kache missing from PATH

With the action fixed, the build died:

error: could not execute process `kache /home/alex/.rustup-runner-5/toolchains/1.96.../bin/rustc -vV` (never executed)
Caused by: No such file or directory (os error 2)

The host sets rustc-wrapper = "kache" in ~/.cargo/config.toml and the kache daemon runs, but the binary lives in ~/.local/bin, which the terraphim-ai runner .env omits:

PATH=/home/alex/.cargo-runner-5/bin:/home/alex/.rustup-runner-5/toolchains/stable-.../bin:/usr/local/bin:/usr/bin:/bin

The terraphim-ai runners are configured without that directory; the wrapper therefore cannot be found even though the binary is installed and the daemon runs.

Fix the PATH, not the wrapper. Adding ~/.local/bin via GITHUB_PATH lets the existing wrapper resolve and keeps kache's content-addressed cache. Clearing RUSTC_WRAPPER would also get past the error but would discard the cache the wrapper exists to provide, slowing every self-hosted build. That approach was tried and rejected.

Change

- name: Install Rust
  run: |
    rustup toolchain install 1.96 --profile minimal --component rustfmt,clippy
    rustc --version
    cargo --version

- name: Put the kache build cache on PATH
  run: |
    echo "${HOME}/.local/bin" >> "$GITHUB_PATH"
    "${HOME}/.local/bin/kache" --version

Verification

Dispatched the workflow on this branch and the job ran green end to end:

Step	Result
Install Rust	success
Put the kache build cache on PATH	success
Build adf binary	success
Run agent validation (all modes)	success
Upload validation report	success
Parse and display results	success

Before the fix, the action step failed within seconds. Two intermediate runs pinned down the second fault: 37978805868 (build failed on kache) and 37979446951 (my initial PATH step called kache before GITHUB_PATH took effect — that applies from the next step).

Mirrored

terraphim/terraphim-ai#3478 on Gitea carries the same squashed change; the file is identical on both lines.

Follow-up not in scope

terraphim/gitea-robot #70 reports the same class of problem wider: workflows request ubuntu-latest while only terraphim-native is registered. Separately, other workflows on these same runners may hit the kache PATH fault; a per-workflow PATH step is the portable fix, since the runner .env is managed outside the repo.