avoid ENOENT error when rebuilding deps

Without this fix, I frequently get the following error:
# This file contains the result of Yarn building a package (rescript-relay@virtual:d9ee19c7d21b51ab446d207037b2e76a866dce1033d055f68eea718d1aad4e8e07b9ebe79e1234ad68e9c3b88e667d22ab495f68c31bf24afd238dcea56f49b1#npm:3.0.0-rc.1)
# Script name: postinstall

node:fs:3003
  binding.copyFile(
          ^

Error: ENOENT: no such file or directory, copyfile '/Users/paul/code/miriad/packages/web/node_modules/rescript-relay/ppx-macos-latest' -> '/Users/paul/code/miriad/packages/web/node_modules/rescript-relay/ppx'
    at Object.copyFileSync (node:fs:3003:11)
    at copyPlatformBinaries (/Users/paul/code/miriad/packages/web/node_modules/rescript-relay/postinstall.js:102:6)
    at Object.<anonymous> (/Users/paul/code/miriad/packages/web/node_modules/rescript-relay/postinstall.js:177:7)
    at Module._compile (node:internal/modules/cjs/loader:1376:14)
    at Module._extensions..js (node:internal/modules/cjs/loader:1435:10)
    at Module.load (node:internal/modules/cjs/loader:1207:32)
    at Module._load (node:internal/modules/cjs/loader:1023:12)
    at Function.executeUserEntryPoint [as runMain] (node:internal/modules/run_main:135:12)
    at node:internal/main/run_main_module:28:49 {
  errno: -2,
  code: 'ENOENT',
  syscall: 'copyfile',
  path: '/Users/paul/code/miriad/packages/web/node_modules/rescript-relay/ppx-macos-latest',
  dest: '/Users/paul/code/miriad/packages/web/node_modules/rescript-relay/ppx'
}

Node.js v20.10.0
zth / rescript-relay

avoid ENOENT error when rebuilding deps #487
