join() was broken

forgot -m for pip

src/tasks/python.cpp

@@ -248,7 +248,7 @@ void python::install_pip()
 	// ssl errors while downloading through python without certifi
 	run_tool(process_runner(process()
 		.binary(python_exe())
-		.arg("pip")
+		.arg("-m pip")
 		.arg("install")
 		.arg("certifi")));
 }

src/utility.h

@@ -323,7 +323,7 @@ T join(const std::vector<T>& v, const Sep& sep)
 
 	for (auto&& e : v)
 	{
-		if (first)
+		if (!first)
 			s += sep;
 
 		s += e;