Write shell completions to nicely available directory like man pages
Currently, sequoia's CLI utilities all have the ability to generate shell completions during compile time using their build.rs files. ([sq](https://gitlab.com/sequoia-pgp/sequoia/-/blob/c244c4e256eb5c712f610e95f868313c621267c4/sq/build.rs#L36-45), [sqv](https://gitlab.com/sequoia-pgp/sequoia-sqv/-/blob/fbb49393c3ea95effe0f7be1b349808af90f65cf/build.rs#L29-38), [sop](https://gitlab.com/sequoia-pgp/sequoia-sop/-/blob/80c1ddd6594ca96c0809445c698d9fd18cc8568a/build.rs#L3-6)). They all require setting the `CARGO_TARGET_DIR` variable to enable writing out the completion files, which changes where cargo will put other generated artifacts as well: https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-reads I think it'd make sense to write out shell completions to a separate directory instead, something like `completions-${command_name}`, similar to how man pages are put into `man-sq` etc. This would make packaging `sq`, `sqv` and `sqop` completely, including the completions, quite a bit easier.
issue