Fix updating credentials on different mounts/drives/volumes

[quinchs]

Summary

This PR fixes #1330 by ensuring the temp file created for database in the project config is created in the same mount/drive/volume as its original file, permitting a rename.

[scotttrinh]

Changes look good to me!

[aljazerzen]

This does not fix the linked issue.

Here is some debugging info:

--- src/branch/context.rs:127 ---
                dbg!(&stash_path);
                let tmp = tmp_file_name(&stash_path);
                dbg!(&tmp);
                fs::write(&tmp, branch)?;
                fs::rename(&tmp, &stash_path.join("database"))?;

➜  edgedb-project2 pwd
/run/media/aljaz/USB/edgedb-project2
➜  edgedb-project2 edgedb branch switch x
Switching from 'main' to 'x'
[src/branch/context.rs:127:17] &stash_path = "/home/aljaz/.config/edgedb/projects/edged
b-project2-65ffeee7269f9567c19e74dd40460d99624093a3"
[src/branch/context.rs:129:17] &tmp = ".~edgedb-project2-65ffeee7269f9567c19e74dd40460d
99624093a3.tmp"
edgedb error: Invalid cross-device link (os error 18)

[aljazerzen]

Ideally, we'd have a test for this somewhere (probably in docker) and host the project on a tmpfs.

[aljazerzen]

I can confirm the bug is now fixed.