This fixes a bug with fish completions. Before this commit,
`borgmatic --fish-completion` would generate a fish completion script
with an expanded XDG_CONFIG_HOME (if this variable is set).
This is problematic for package maintainers if the variable is set
during the creation of a package.
Moreover, it is problematic if the user has set this variable because
the fish completion scripts checks its own consistency with `borg
--fish-completion` during startup to detect version mismatches. If the
user has a packaged fish completion file with "~/.config/$HOME" in it,
this won't match the output of `borg --fish-completion` because the
latter contains the expansion of $XDG_CONFIG_HOME. As a result, the
script will incorrectly conclude that it is outdated.