speedup build by only building sub directories (#62)

nix/default.nix

@@ -56,6 +56,7 @@ in
     CARGO_BUILD_INCREMENTAL = "false";
     RUST_BACKTRACE = "full";
     copyLibs = true;
+    buildAndTestSubdir = if dontBuildPlugins then name else null;
 
     postInstall = ''
       wrapProgram $out/bin/anyrun \

nix/plugins/default.nix

@@ -40,6 +40,7 @@ in
     RUST_BACKTRACE = "full";
     copyLibs = true;
     cargoBuildFlags = ["-p ${name}"];
+    buildAndTestSubdir = "plugins/${name}";
 
     meta = with lib; {
       description = "The ${name} plugin for Anyrun";