Remove manual update of nix crate in audit workflow

.github/workflows/audit.yml

@@ -8,8 +8,5 @@ jobs:
     - name: Install cargo-audit
       run: cargo install cargo-audit
     - uses: actions/checkout@master
-    - name: Manually update `nix` crates # See https://github.com/nix-rust/nix/issues/1627
-      run: |
-        cargo update --package nix:0.20.0 --precise 0.20.2
     - name: Audit dependencies
       run: cargo audit