installing gitui releases from source results in a "nightly" version string by default

Running `cargo install gitui` as documented here:
https://github.com/extrawurst/gitui?tab=readme-ov-file#cargo-install

Results in `gitui --version` returning:
`gitui nightly 2024-06-02 ()` (replace `2024-06-02` with the current date)

I'm also having trouble making this work sensibly for the Fedora Linux package for gitui.

I can easily set `GITUI_RELEASE=1`, but then the `get_git_hash()` function from build.rs will fail hard (with an `std::io::Error: No such file or directory`) if `git` is not installed in the build environment. And *if* `git` is available, it will either return `""` (in the case where the git command fails with no output on stdout) or random garbage if there is an unrelated git repository in any of the parent directories.

Would you accept a PR that adapted build.rs to work better in more cases, for example, when `git` is not available?