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:
- os.remove(filepath) # PermissionError/OSError on Win32 if in use
- os.rename(tmp, filepath)
+ if os.path.exists(filepath):
+  os.remove(filepath) # PermissionError/OSError on Win32 if in use
  except OSError:
  shutil.copy2(tmp, filepath)
  os.remove(tmp)
+ return
+ os.rename(tmp, filepath)
 
 
 class RustBuild(object):