melpaBuild: get Emacs package name from recipe

When building a package from a Melpa recipe file, get the Emacs package
name from the recipe. Nix is more restrictive about packages names than
Emacs, so the Nix name for a package is sometimes different.

pkgs/build-support/emacs/melpa.nix

@@ -54,7 +54,7 @@ import ./generic.nix { inherit lib stdenv emacs texinfo; } ({
       else
       ''
       -f melpa2nix-build-package-from-recipe \
-      ${pname} ${version} ${recipeFile}
+      ${recipeFile} ${version}
       ''}
 
     runHook postBuild

pkgs/build-support/emacs/melpa2nix.el

@@ -25,8 +25,10 @@
   (if (not noninteractive)
       (error "`melpa2nix-build-package' is to be used only with -batch"))
   (pcase command-line-args-left
-    (`(,package ,version ,recipe-file)
-     (let* ((rcp (cdr (package-build--read-from-file recipe-file)))
+    (`(,recipe-file ,version)
+     (let* ((recipe (package-build--read-from-file recipe-file))
+            (rcp (cdr recipe))
+            (package (car recipe))
             (files (package-build--config-file-list rcp)))
        (melpa2nix-package-build-archive package version files)))))