Detect HerculesCI, make our nixpkgs version easily available

default.nix

@@ -1,6 +1,6 @@
 # Note: This is just a minimal example. For proper usage, see the README.
 
-{ nixpkgs ? (import <nixpkgs> {}).pkgsMusl, compiler ? "ghc864", strip ? true }:
+{ nixpkgs ? (import ./nixpkgs.nix).pkgsMusl, compiler ? "ghc864", strip ? true }:
 
 
 let

nixpkgs.nix

@@ -0,0 +1,10 @@
+# Detect HerculesCI by checking if NIX_PATH is empty.
+# See https://docs.hercules-ci.com/hercules-ci/getting-started/repository/
+# If it's empty, we give our custom nixpkgs version;
+# otherwise we use what the user has set with NIX_PATH.
+if builtins.getEnv "NIX_PATH" == ""
+  then
+    builtins.trace "NIX_PATH is not set, we're probably in HerculesCI"
+      (import (fetchTarball https://github.com/nh2/nixpkgs/archive/a2d7e9b875e8ba7fd15b989cf2d80be4e183dc72.tar.gz) {})
+  else
+    import <nixpkgs> {}