packaging: fix static build

Normally `pkgsStatic` adds ` -static` to `NIX_CFLAGS_COMPILE`. Due to a bug
this did not apply with `__structuredAttrs`. As the fix [1] has not been
backported yet, put it in the package manually.

[1] https://github.com/NixOS/nixpkgs/pull/428430

Fixes: https://git.lix.systems/lix-project/lix/issues/962

Change-Id: I6a6a6964c6a33f486ba3df3be16f715ad1b060c3

package.nix

@@ -386,6 +386,9 @@ stdenv.mkDerivation (finalAttrs: {
     }
     // lib.optionalAttrs hostPlatform.isLinux {
       BUILD_TEST_SHELL = "${pkgsStatic.busybox}/bin";
+    }
+    // lib.optionalAttrs hostPlatform.isStatic {
+      NIX_CFLAGS_COMPILE = " -static";
     };
 
   cargoDeps = rustPlatform.importCargoLock { lockFile = ./Cargo.lock; };