Add macOS to CI matrix (#68)

Also, re-enable fsatrace in default.nix but only on Linux.

.github/workflows/ci.yaml

@@ -3,12 +3,15 @@ on:
   pull_request:
   push:
 jobs:
-  tests:
-    runs-on: ubuntu-latest
+  build:
+    runs-on: ${{ matrix.os }}
+    strategy:
+      matrix:
+        os: [macos-latest, ubuntu-latest]
     steps:
     - uses: actions/checkout@v1
-    - uses: cachix/install-nix-action@v3
-    - uses: cachix/cachix-action@v2
+    - uses: cachix/install-nix-action@v6
+    - uses: cachix/cachix-action@v3  # This also runs nix-build.
       with:
         name: srid
         signingKey: '${{ secrets.CACHIX_SIGNING_KEY }}'

default.nix

@@ -72,11 +72,16 @@ haskellPackages.developPackage {
   };
   modifier =
     let
+      platformSpecificDeps =
+        if builtins.currentSystem == "x86_64-linux"
+        then [pkgs.fsatrace]
+        else []; # fsatrace disabled on macOS, and requires sytem configuration.
       addExtraDeps =
         (t.flip h.addBuildTools) (with haskellPackages;
           [ cabal-install
             ghcid
-          ]);
+          ] ++ platformSpecificDeps
+        );
     in (t.flip t.pipe) [
       addExtraDeps
       h.dontHaddock