Fix guest Nix flake failures in Git repos on the host filesystem

[midchildan]

Inside the guest, Nix operations fail on flakes in Git repositories located on the host filesystem due to differing UIDs. The changes in this PR fixes this.

Before:

# nix build .#legacyPackages.aarch64-linux.hello
error:
       … while fetching the input 'git+file:///Users/midchildan/Documents/src/repos/github.com/NixOS/nixpkgs'

       error: opening Git repository "/Users/midchildan/Documents/src/repos/github.com/NixOS/nixpkgs": repository path '/Users/midchildan/Documents/src/repos/github.com/NixOS/nixpkgs' is not owned by current user

After:

# nix build .#legacyPackages.aarch64-linux.hello
# result/bin/hello
Hello, world!