improve: `sha256sum`-referenced files should be relative to the `sha256sum` file itself (#2963)

* improve: `sha256sum`-referenced files should be relative to the `sha256sum` file itself

`cd` to the directory before generating the files

Fixes: https://github.com/flameshot-org/flameshot/issues/2878

* Use `tee` instead of double-invocation (`coreutils` should be standard on any installation?)