Add String functions to filesystem implementations

[inteon]

In error messages, filesystems are often printed (eg.

apko/pkg/apk/apk/repo.go

Line 129 in db9be70

return nil, fmt.Errorf("could not open repositories file in %s at %s: %w", a.fs, reposFilePath, err)

).

Without String function, this resulted in unreadable error messages:

Error: could not open repositories file in &{/ %!s(*fs.memFS=&{0xc000162000}) map[] {{} {%!s(int32=0) %!s(uint32=0)}}} at etc/apk/repositories: open /etc/apk/repositories: no such file or directory

This is the new error message:

Error: could not open repositories file in dirfs:/ at etc/apk/repositories: open /etc/apk/repositories: no such file or directory

[inteon]

We could potentially add this function to the FullFS interface, but this would be a breaking change, WDYT?