Don't give an error when creating a file for the first time

Previously, `os.remove` would always give a FileNotFound error the first
time you called it, causing bootstrap to make unnecessary copies. This
now only calls `remove()` if the file exists, avoiding the unnecessary
error.

src/bootstrap/bootstrap.py

@@ -351,11 +351,13 @@ def output(filepath):
     with open(tmp, 'w') as f:
         yield f
     try:
+        if os.path.exists(filepath):
             os.remove(filepath)  # PermissionError/OSError on Win32 if in use
-        os.rename(tmp, filepath)
     except OSError:
         shutil.copy2(tmp, filepath)
         os.remove(tmp)
+        return
+    os.rename(tmp, filepath)
 
 
 class RustBuild(object):