CI: set PATH in the workflow; add a toolchain diagnostic step

.github/workflows/ci.yml

@@ -9,6 +9,11 @@ on:
 env:
   CARGO_TERM_COLOR: always
   RUSTFLAGS: -D warnings
+  # The runner's .path/.env files are not reliably applied, so PATH is set
+  # here where it demonstrably is. /opt/rust holds the isolated toolchain.
+  PATH: /opt/rust/cargo/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
+  RUSTUP_HOME: /opt/rust/rustup
+  CARGO_HOME: /opt/rust/cargo
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}
@@ -39,6 +44,11 @@ jobs:
     runs-on: [self-hosted, Linux, ARM64, spark-1]
     steps:
       - uses: actions/checkout@v5
+      - name: toolchain
+        run: |
+          echo "PATH=$PATH"
+          command -v cargo || echo "cargo NOT on PATH"
+          cargo --version
       - run: cargo fmt --all --check
       - run: cargo clippy --workspace --all-targets --all-features
       - run: cargo test --workspace --all-features