fix installing zig when no project is open

This whole execCmd stuff is borrowed from somewhere
else and should probably be removed.

Closes #125

src/zigUtil.ts

@@ -54,7 +54,7 @@ export function execCmd
         const cmdArguments = options ? options.cmdArguments : [];
 
         childProcess =
-            cp.execFile(cmd, cmdArguments, { cwd: detectProjectRoot(fileName || workspace.rootPath + "/fakeFileName"), maxBuffer: 10 * 1024 * 1024 }, handleExit);
+            cp.execFile(cmd, cmdArguments, { cwd: detectProjectRoot(fileName || ""), maxBuffer: 10 * 1024 * 1024 }, handleExit);
 
 
         childProcess.stdout.on("data", (data: Buffer) => {