Edit github workflow to run audit on latest stable only

cargo-audit now has an MSRV of 1.74 > ours. We only need to run audit on
at least one toolchain, anyhow.

.github/workflows/build.yml

@@ -50,4 +50,4 @@ jobs:
 
       - name: Run cargo audit
         run: |
-          [[ ${{ matrix.toolchain }} == nightly ]] || (cargo install cargo-audit && cargo audit)
+          [[ ${{ matrix.toolchain }} != 1.80.0 ]] || (cargo install cargo-audit && cargo audit)