Remove the need to hardcode absolute paths for some entries in `.cabal/config`: support xdg home variables

Hi!

I keep all my configuration files on github to have an easily reproducible environment on all my machines. I keep my `.cabal/config` file in there as well, and don't like the fact that some absolute paths have to be hardcoded (e.g. `remote-repo-cache`, `world-file` and `build-summary`). This is inconvenient if `$HOME` happens to be different on various machines (e.g. work vs home).

Some possible solutions (which could be wrong):
- Expand `~` in paths.
- Expand environment variables from the shell (perhaps even very specific ones)
- Allow usage of `$prefix` in all paths

If one of the above suggestions is worthwhile, I'd be up for implementing it. Just let me know :smile:

Thank you!

Start by reviewing how .cabal/config handles remote-repo-cache, world-file, and build-summary paths. Compare the proposed ~, shell environment variables, and $prefix forms, then determine which path-expansion behavior is appropriate. Done means the selected form works without machine-specific absolute paths and is documented and tested.

Written by the indexing model from the issue text.