[nix] Replaces brew install of watchman with nix

Summary:
Because we updated nix packages, we can use nix-based watchman instead of the brew install

Context: https://linear.app/comm/issue/ENG-2784/add-updated-nix-version-of-watchman#comment-79fd27f8

Test Plan:
```
 ➜ whereis watchman
watchman: /nix/store/pmvk9zd5z98b5vvc83w4dqjq7d4w8ri1-watchman-2023.08.14.00/bin/watchman
```

To verify the new Nix watchman is working. I uninstalled brew-based watchman and ran nix develop. I then ran the iOS simulator and made changes to text on the login screen, verifying that changes were immediate

Reviewers: ashoat, bartek, varun, O1 Nix

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D12777

nix/dev-shell.nix

@@ -40,6 +40,7 @@
 , sops
 , sqlite
 , terraform
+, watchman
 , rustfmt
 , wasm-pack
 , yarn
@@ -65,6 +66,7 @@ mkShell {
     mariadb
     nodejs
     yarn
+    watchman # react native
     python3
     redis
     wasm-pack
@@ -141,8 +143,6 @@ mkShell {
     wait "$mariadb_pid" "$redis_pid"
 
     ${../scripts}/install_homebrew_macos.sh
-
-    ${../scripts}/install_homebrew_deps.sh watchman
   '' + ''
 
     # Render default configuration for keyserver