xo-unit: + ./pkgs/xo-userenv for nix CI

pkgs/xo-userenv.nix

@@ -0,0 +1,24 @@
+{
+  # nixpkgs dependencies
+  buildFHSUserEnv, # ... other deps here
+
+  # xo dependencies
+  xo-cmake, xo-indentlog, xo-subsys, xo-refcnt, xo-reflect, xo-unit,
+
+  # other args
+
+  # someconfigurationoption ? false
+} :
+
+buildFHSUserEnv {
+  name = "xo-userenv";
+  targetPkgs = pkgs: [ xo-cmake
+                       xo-indentlog
+                       xo-subsys
+                       xo-refcnt
+                       xo-reflect
+                       xo-unit
+                     ];
+  # runScript = ...;
+  # profile = ...;
+}