Fix "~/xpk" references

I tried making it `os.path.join(os.path.expanduser("~"), "directory")` in one of my earlier commits but that doesn't work. Explicitly someone made a mistake earlier on by not expanding `~` with [`os.path.expanduser`](https://docs.python.org/3/library/os.path.html#os.path.expanduser) so it actually expects in the maxtext directory a folder named `~` with the `xpk` folder inside. Otherwise the tests fail on CI.

_Originally posted by @SamuelMarks in https://github.com/AI-Hypercomputer/maxtext/pull/1560#discussion_r2037944071_