ref(bao_recipe): update local/remote srcs assignment

[Diogo21Costa]

PR Description

This PR refactors the logic for assigning the srcs attribute in the Bao Nix recipe.

The recipe is typically used with the following syntax:

bao = callPackage (nix_pkgs_path + "/bao/bao.nix")            { 
              toolchain = aarch64-none-elf;
              bao_srcs_path = /path/to/bao/repo;
              inherit bao_cfg_repo;
              inherit bao_cfg;
              platform_cfg = plat_cfg;
              inherit guests;
            };

If bao_srcs_path is defined, the build of Bao uses local sources. If bao_srcs_path is not defined, the code is fetched from GitHub, and remote sources are used.

This change allows a single nix recipe for building the hypervisor, either local or remote sources.

Please provide your feedback on these changes.