Improve the error message if `.heroku` is a file not a directory

eg:

```
-----> Building on the Heroku-24 stack
-----> Determining which buildpack to use for this app
-----> Python app detected
-----> Using Python 3.11 specified in .python-version
-----> Installing Python 3.11.11
mkdir: cannot create directory ‘/tmp/build_6c33c932/.heroku’: Not a directory
 !     Push rejected, failed to compile Python app.
```

GUS-W-19247001.