Fix nix reproducible build

[dvcorreia]

Having the build date on the binary makes the omlox-cli binary not reproducible.

This removes the build date and adds the nix derivation and overlay for omlox-cli.

Other things to note:

• copywrite is now available in the upstream nixpkgs, so is was removed
• dependency on numtide/flake-utils was dropped

[cfradewavecom]

BUILD_DATE exists so anyone can identify wrongly or by mistake local changes where made, COMMIT_HASH is not enough. Is there another valid reason to justify this change?

[dvcorreia]

BUILD_DATE exists so anyone can identify wrongly or by mistake local changes where made, COMMIT_HASH is not enough. Is there another valid reason to justify this change?

You're right that having some form of date information is useful. However, BUILD_DATE doesn't actually tell us about code changes, only when it was compiled. If you'd like to keep some kind of date information, I'd suggest using the commit date instead, which provides the same benefit while maintaining reproducibility since it doesn't change between builds of the same commit.

nix build already introduces a dirty flag if the git state is dirty.

> nix build
warning: Git tree '/Users/dvcorreia/Desktop/projects/omlox-client-go' is dirty
> ./result/bin/omlox-cli version
omlox version 9b5b799-dirty (9b5b799bbdcf56c435738c3f357305d1763b812a-dirty)

If this help avoid confusion introduced by local changes, wouldn't be hard to extend this concept to the Makefile.

On the topic of reproducible builds, they have several benefits.
I would say that the most important in this case is verifying that the binaries you download match the original, untampered source code.

This verification extends beyond Nix. A good example is OCI images, where we can specify a content digest to make the image verifiable, avoiding some supply chain attack vectors. If the binary isn't reproducible, we can't ensure the image wasn't tampered with, since every build changes the digest.

It's not a critical issue, but it's a valuable property that improves security and developer experience. Being able to install things through Nix without these concerns provides a better experience for users trying omlox-cli.

[cfradewavecom]

lgtm