Add support to build with musl

Add support to the flake.nix to build with musl
```
nix build .#patchelf-musl
```

Author: fzakaria

.gitignore

@@ -36,3 +36,4 @@ Makefile
 .direnv/
 .vscode/
 .idea/
+result

flake.nix

@@ -21,6 +21,10 @@
 
     {
       overlay = final: prev: {
+        patchelf-new-musl = final.pkgsMusl.callPackage ./patchelf.nix {
+          inherit version;
+          src = self;
+        };
         patchelf-new = final.callPackage ./patchelf.nix {
           inherit version;
           src = self;
@@ -85,12 +89,19 @@
         build = self.hydraJobs.build.${system};
       });
 
+      devShell = forAllSystems (system: pkgs.mkShell {
+          buildInputs = [ self.defaultPackage.${system}.inputDerivation];
+      });
+
       defaultPackage = forAllSystems (system:
-        (import nixpkgs {
-          inherit system;
-          overlays = [ self.overlay ];
-        }).patchelf-new
+        self.packages.${system}.patchelf
       );
 
+      packages = forAllSystems (system:
+        {
+          patchelf = nixpkgsFor.${system}.patchelf-new;
+          patchelf-musl = nixpkgsFor.${system}.patchelf-new-musl;
+        });
+
     };
 }