fix(ci): override RUSTFLAGS for audit job to fix stable toolchain compatibility (#439)

The global RUSTFLAGS env var sets -Zthreads=32 which is a nightly-only flag.
This breaks the security audit job which uses stable Rust to install and run
cargo-audit. The fix overrides RUSTFLAGS to empty string for the audit job,
allowing cargo-audit to be installed with the stable toolchain.

Co-authored-by: Droid Agent <droid@factory.ai>

Author: echobt

.github/workflows/ci.yml

@@ -270,6 +270,9 @@ jobs:
     permissions:
       contents: read
       issues: write
+    # Override global RUSTFLAGS - the -Zthreads flag is nightly-only and breaks cargo-audit installation on stable
+    env:
+      RUSTFLAGS: ""
     steps:
       - uses: actions/checkout@v4