Re-write GitHub action to improve reliability

Re-write the GitHub action to work more like Hydra

* Packages are evaluated and then built in topological order
* Each build failure is added as a separate cache entry. Makes it work
  even if the action times out, and makes it easier to retry individual
  packages.
* Dependency failures are reported accurately

.github/actions/nix-ros-build-action/.gitignore

@@ -11,3 +11,6 @@ Thumbs.db
 # Ignore built ts files
 __tests__/runner/*
 lib/**/*
+
+# Output directory
+build/

.github/actions/nix-ros-build-action/action.yml

@@ -10,17 +10,17 @@ inputs:
     description: Root attribute to begin evaluation
     required: true
     default: rosPackages
-  nixpkgs:
-    description: URL used to download nixpkgs
-    required: true
-    default: https://github.com/lopsided98/nixpkgs/archive/nix-ros.tar.gz
   system:
     description: Host system name (e.g. x86_64-linux). If different from the runner architecture, binfmt/qemu must be configured.
     required: false
-  parallelism:
+  eval-jobs:
+    description: Number of evaluations to run in parallel
+    required: true
+    default: "4"
+  build-jobs:
     description: Number of builds to run in parallel
     required: true
-    default: "20"
+    default: "2"
   cachix-cache:
     description: Name of the Cachix cache to use
     required: true

.github/actions/nix-ros-build-action/eval.nix

@@ -1,4 +1,22 @@
-{ lib, pkgs }: let
-  validDrv = (n: v: (builtins.tryEval v).success);
-  listDrvs = prefix:
-in lib.attrNames (lib.filterAttrs validDrv pkgs)
+# Evaluate `release.nix' like Hydra would. Too bad nix-instantiate can't to do this.
+{ lib ? pkgs.lib, pkgs }:
+
+let
+  trace = if builtins.getEnv "VERBOSE" == "1" then builtins.trace else (x: y: y);
+
+  # Add the ‘recurseForDerivations’ attribute to ensure that
+  # nix-instantiate recurses into nested attribute sets.
+  recurse = path: attrs:
+    if (builtins.tryEval attrs).success then
+      if lib.isDerivation attrs
+      then
+        if (builtins.tryEval attrs.drvPath).success
+        then { inherit (attrs) name drvPath; }
+        else { failed = true; }
+      else if lib.isAttrs attrs && (attrs.recurseForDerivations or false) then
+        { recurseForDerivations = true; } //
+           lib.mapAttrs (n: v: let path' = path ++ [n]; in trace path' (recurse path' v)) attrs
+      else { }
+    else { };
+
+in recurse [] pkgs