fix: stabilize gosec lint concurrency for release

[RyshMan]

Problem

• release preflight on macOS hit a nondeterministic gosec panic (fatal error: concurrent map read and map write) during make prepush-full
• the crash blocked cutting the next release from main

Root cause

• gosec v2.23.0 runs with default concurrency 8, which proved flaky under the current local toolchain during repo-wide scans

Fix

• run gosec single-threaded from the Makefile via a GOSEC_CONCURRENCY knob defaulted to 1
• match the same -concurrency 1 invocation in GitHub Actions lint so local and CI behavior stay aligned

Validation

• bash scripts/test_github_action_runtime_versions.sh
• make lint
• make prepush-full