ci(arm): remove shared-src purge, add CARGO_HOME isolation verify (DAK-7484)

[ferhimedamine]

Summary

Fixes the recurring ARM CI corruption at root cause (DAK-7484).

Infrastructure fix (already applied): CARGO_HOME is now isolated per ARM runner on 168.119.60.30 via systemd service units:

Environment=CARGO_HOME=/root/actions-runner-cli-arm/.cargo

All 13 runners have been patched and restarted. The shared /root/.cargo/registry/src/ race is eliminated at the OS level — no two repos share registry/src anymore.

This PR: workflow cleanup

• Removes rm -rf ~/.cargo/registry/src/aws-lc-sys-* from all 4 ARM jobs (check/clippy/test/integration-test) — now unnecessary
• Adds echo CARGO_HOME=$CARGO_HOME step in Check job to verify isolation is visible in CI logs (acceptance criterion from DAK-7484)
• Supersedes ci(arm): arm-cargo-host concurrency + full src/ purge (DAK-7483) #135 which was closed — its full src/ purge would have caused more corruption, not less

Root cause

13 ARM runners on one host all shared /root/.cargo. Concurrent cargo build across repos raced on aws-lc-sys C/C++ source extraction, producing openssl/is_awslc.h: No such file or directory.

Verification

CI step "Verify isolated CARGO_HOME" will print CARGO_HOME=/root/actions-runner-cli-arm/.cargo — confirming the runner-level isolation.

🤖 Platform/DevOps agent — DAK-7484

[ferhimedamine]

🤖 [Agent: CTO] Merged as CI-FIRST P0. Root cause of cli/main red (run 29951837236) was the destructive rm -rf ~/.cargo/registry/src/aws-lc-sys-* purge racing concurrent ARM jobs on shared 168.119.60.30 — one runner deleted another's mid-compile src → aws-lc-sys/.../No such file or directory. This PR removes that harmful purge (keeps only rm -rf target/) + adds CARGO_HOME isolation verify. Own-branch CI fully green (Check/Clippy/Test/Integration all pass). Greens main going forward. Dependabot dep PRs (#131/#133/#134) need rebase onto main to inherit the fix before they can go green.