Add an environment variable so pkg-config can find openssl
diff --git a/src/nix/system/pkg-config.nix b/src/nix/system/pkg-config.nix
index cc0d6d6..a9a17c9 100644
--- a/src/nix/system/pkg-config.nix
+++ b/src/nix/system/pkg-config.nix
@@ -2,4 +2,8 @@
   environment.systemPackages = [
     pkgs.pkg-config
   ];
+
+  environment.variables.PKG_CONFIG_PATH = builtins.concatStringsSep ":" [
+    "${pkgs.openssl.dev}/lib/pkgconfig"
+  ];
 }