Make used temporary directory user-specific

[mforthewin]

In a multi-user setup, the neovim-page temporary directory leads to problems, as the second user will not have the necessary permissions for it to work, leading to a panic:

thread 'main' panicked at .../page-git/src/page/src/connection.rs:339:5:
Cannot connect to neovim: attempts=81, address="/tmp/neovim-page/socket-<...>", Os { code: 2, kind: NotFound, message: "No such file or directory" }
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

This PR fixes the panic by utilizing UID-specific temporary directories with fitting permissions.