checks: generalize and make them more configurable

All the checks are now aggregated in system.checks.text making it easy
to allow certain checks to be disabled as well as disabling them
alltogether if desired. eg.

    # Disable all checks.
    system.checks.text = mkForce "";

Fixes #117

modules/system/checks.nix

@@ -3,6 +3,8 @@
 with lib;
 
 let
+  cfg = config.system.checks;
+
   darwinChanges = ''
     darwinChanges=/dev/null
     if test -e /run/current-system/darwin-changes; then
@@ -30,7 +32,7 @@ let
     fi
   '';
 
-  buildUsers = optionalString config.services.nix-daemon.enable ''
+  buildUsers = ''
     buildUser=$(dscl . -read /Groups/nixbld GroupMembership 2>&1 | awk '/^GroupMembership: / {print $2}') || true
     if [ -z $buildUser ]; then
         echo "error: Using the nix-daemon requires build users, aborting activation" >&2
@@ -135,7 +137,7 @@ let
     fi
   '';
 
-  nixGarbageCollector = optionalString config.nix.gc.automatic ''
+  nixGarbageCollector = ''
     if test -O /nix/store; then
         echo "error: A single-user install can't run gc as root, aborting activation" >&2
         echo "Configure the garbage collector to run as the current user:" >&2
@@ -149,19 +151,34 @@ in
 
 {
   options = {
+    system.checks.verifyNixPath = mkOption {
+      type = types.bool;
+      default = true;
+      description = "Whether to run the NIX_PATH validation checks.";
+    };
+
+    system.checks.text = mkOption {
+      internal = true;
+      type = types.lines;
+      default = "";
+    };
   };
 
   config = {
 
+    system.checks.text = mkMerge [
+      darwinChanges
+      runLink
+      (mkIf config.services.nix-daemon.enable buildUsers)
+      nixStore
+      (mkIf config.nix.gc.automatic nixGarbageCollector)
+      nixChannels
+      nixInstaller
+      (mkIf cfg.verifyNixPath nixPath)
+    ];
+
     system.activationScripts.checks.text = ''
-      ${darwinChanges}
-      ${runLink}
-      ${buildUsers}
-      ${nixStore}
-      ${nixGarbageCollector}
-      ${nixChannels}
-      ${nixInstaller}
-      ${nixPath}
+      ${cfg.text}
 
       if test ''${checkActivation:-0} -eq 1; then
         echo "ok" >&2