Build.zig: Skip copying files that are used internally by the compiler

Before this it was trying to copy all the files from the zig-cache dir
to the output dir, but in the current compiler architecture the cache
dir is also used for internal compiler files.

lib/std/build.zig

@@ -2331,6 +2331,14 @@ pub const LibExeObjStep = struct {
 
                 var it = src_dir.iterate();
                 while (try it.next()) |entry| {
+                    // The compiler can put these files into the same directory, but we don't
+                    // want to copy them over.
+                    if (mem.eql(u8, entry.name, "stage1.id") or
+                        mem.eql(u8, entry.name, "llvm-ar.id") or
+                        mem.eql(u8, entry.name, "libs.txt") or
+                        mem.eql(u8, entry.name, "builtin.zig") or
+                        mem.eql(u8, entry.name, "lld.id")) continue;
+
                     _ = try src_dir.updateFile(entry.name, dest_dir, entry.name, .{});
                 }
             } else {