Otherwise you end up with two binaries, only one of which is stripped.
This causes an extra 4.3MB, because 'make app' does another cargo build
and copies that binary into the .app.

Motivation for this change

Installing alacritty on darwin right now causes two binaries to be created, only one of which is stripped.

tree -h /n/s/zqyw2yx0j4w5b6syh0adf79xm5wdb18l-alacritty-0.4.3
.
├── [  96]  Applications
│   └── [  96]  Alacritty.app
│       └── [ 160]  Contents
│           ├── [2.4K]  Info.plist
│           ├── [  96]  MacOS
│           │   └── [4.3M]  alacritty
│           └── [  96]  Resources
│               └── [650K]  alacritty.icns
├── [  96]  bin
│   └── [3.7M]  alacritty
├── [  96]  nix-support
│   └── [  69]  propagated-user-env-packages
└── [ 192]  share
    ├── [  96]  bash-completion
    │   └── [  96]  completions
    │       └── [1.8K]  alacritty.bash
    ├── [  96]  fish
    │   └── [  96]  vendor_completions.d
    │       └── [1.9K]  alacritty.fish
    ├── [  96]  man
    │   └── [  96]  man1
    │       └── [1.3K]  alacritty.1.gz
    └── [  96]  zsh
        └── [  96]  site-functions
            └── [1.3K]  _alacritty

16 directories, 9 files

In this PR I'm overwriting the binary inside the .app with a link to the binary inside bin/. The build still compiles the binary twice, but preventing that would require a patch to the Makefile

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.