libstore: Use unix-dotfile vfs if useSQLiteWAL is false

[doronbehar]

Fixes #2357 . cc @wmertens @veprbl

I don't know how to test this, since I don't know how to reproduce the sqlite db corruption issue mentioned in https://nixos.wiki/wiki/Nix_Installation_Guide#NFS . I also rarely write C code so this is probably wrong.

I also hope I got the logic correct.

[wmertens]

Well, LGTM in any case :) I think the corruption is pretty hard to do but involves 2 writers, perhaps one on NFS and the other on the filesystem.

[doronbehar]

Thanks for the approval! Backports: #5490 #5491 .

[tomberek]

This commit makes me think there may be some interesting situations where one can query or use sqlite via the HTTP VFS. This can allow interesting or bulk queries to be run efficiently over a sqlite uploaded to S3/static hosting with Range requests.