Compiling on My Mac Using Xcode in Devbox (Nix)

Getting Devbox (or nix) to work with MacOS compile chains can be tricky. In my cse I needed to use an older version of Xcode (15). While there is a nix package for xcode, it will not actually download Xcode. Instead the user has to download it and manually add it to the nix-store. Here's how I did it:

Download Xcode from the Apple Developer site.
Extract the download and add it to the store:
```
open -W Xcode_15.4.xip
nix-store --add-fixed --recursive sha256 Xcode.app
devbox add darwin.xcode_15_4
```
Here, you'll need to keep track of where in the nix-store Xcode has been added. If you forget, you can run the following command from within your Devbox directory:
```
cat .devbox/nix/profile/default/manifest.json | grep -Eio '\/nix\/store\/.*Xcode\.app'
```
Set the DEVELOPER_DIR, SDKROOT, CPATH, LIBRARY_PATH, and PATH environment variables in your devbox.json. In particular, my devbox.json had the following:
```
{
  "env":{
    "DEVELOPER_DIR": "/nix/store/dca1525f56e59ae54d9dd774e14e7a19-Xcode.app/Contents/Developer"
  },
  "shell": {
    "init_hook": [
      "export SDKROOT=$(xcrun --sdk macosx14.5 --show-sdk-path)",
      "export CPATH=$SDKROOT/usr/include",
      "export LIBRARY_PATH=$SDKROOT/usr/lib",
      "export PATH=\"$DEVELOPER_DIR/usr/bin:$DEVELOPER_DIR/Toolchains/XcodeDefault.xctoolchain/usr/bin:$PATH\"",
    ],
    }
}
```
Note that the $DEVELOPER_DIR is the nix-store path of Xcode.app, plus /Contents/Developer.

Also, note that for my $PATH I added $DEVELOPER_DIR/usr/bin and $DEVELOPER_DIR/Toolchains/XcodeDefault.xctoolchain/usr/bin to the front of the path. The Toolchains directory ensures that I use the right compiler (clang et al) for my Xcode version.

That's pretty much it!

In my particular case, running a tauri (rust underneath) app, I also needed to add

{
  "packages": [
    "nodejs@latest",
    "libiconv@latest",
    "rustup@latest",
    "darwin.xcode_15_4@latest"
  ],
  "shell": {
    "init_hook": [
      "projectDir=$(dirname $(readlink -f \"$0\"))",
      "rustupHomeDir=\"$projectDir\"/.rustup",
      "mkdir -p $rustupHomeDir",
      "export RUSTUP_HOME=$rustupHomeDir",
      "rustup default stable",
      "cd src-tauri && cargo fetch"
    ]
  }

}

to my devbox.json