nix: separate fsu from package

This appears to be the only way to build them with different configuration. This enables static linking in the main package.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/fsu/package.nix

@@ -0,0 +1,19 @@
+{
+  buildGoModule,
+  fortify ? abort "fortify package required",
+}:
+
+buildGoModule {
+  pname = "${fortify.pname}-fsu";
+  inherit (fortify) version;
+
+  src = ./.;
+  inherit (fortify) vendorHash;
+  CGO_ENABLED = 0;
+
+  preBuild = ''
+    go mod init fsu >& /dev/null
+  '';
+
+  ldflags = [ "-X main.Fmain=${fortify}/libexec/fortify" ];
+}