add temp CI job to test syspolicy impact

Starting in Catalina, macOS runs a syspolicyd "assessment" that hits the network for each binary/script executable. It does cache these results, but Nix tends to introduce many "new" executables per build. (You can read more about this at NixOS#3789).

This PR adds a temporary, redundant macOS job with these assessments disabled. I'm hoping you can adopt it for a few weeks to help me collect more data on how this affects real projects.

.github/workflows/test.yml

@@ -12,3 +12,13 @@ jobs:
     - uses: actions/checkout@v2
     - uses: cachix/install-nix-action@v10
     - run: nix-build release.nix --arg nix '{ outPath = ./.; revCount = 123; shortRev = "abcdefgh"; }' --arg systems '[ builtins.currentSystem ]' -A installerScript -A perlBindings
+  macos_perf_test:
+    runs-on: macos-latest
+    steps:
+    - name: Disable syspolicy assessments
+      run: |
+        spctl --status
+        sudo spctl --master-disable
+    - uses: actions/checkout@v2
+    - uses: cachix/install-nix-action@v10
+    - run: nix-build release.nix --arg nix '{ outPath = ./.; revCount = 123; shortRev = "abcdefgh"; }' --arg systems '[ builtins.currentSystem ]' -A installerScript -A perlBindings