Manual Flutter Install NixOS, Override Nixpkgs Flutter Version?

If Im using flakes to create the dev environment, I found this post about pinning to a specific commit:

But using the latest commit on unstable (which matches master’s flutter at time of writing) its the same version. I tried looking at the flutter package definition but it is not clear to me how the version of flutter is decided. Is this the approach you mean?
nix flutter pkg