ios build: yarn build on node_modules/react-native/react-native-codegen

We recently started building RN from Git, in 0509575. This caused
an iOS build failure in the shell script '[CP-User] Generate Specs':

  Error: Cannot find module '/Users/chrisbobbe/dev/zulip-mobile/node_modules/react-native/packages/react-native-codegen/lib/cli/combine/combine-js-to-schema-cli.js'

Note the last part:
  [...]/lib/cli/combine/combine-js-to-schema-cli.js'

There is a [...]/src/[...] version of that file, but the /lib/ one
is created by `yarn build`, and when I ran that, the app build
succeeded.

Author: chrisbobbe

tools/postinstall

@@ -7,6 +7,17 @@ patch_package() {
     node_modules/.bin/patch-package
 }
 
+# Since we take react-native from Git, some packages that ship with it
+# and are used in the build need their build scripts run.
+build_rn_packages() {
+    (
+        cd node_modules/react-native/packages/react-native-codegen/
+        # A copy of the package's `yarn build` script
+        # with a `--verbose` flag removed.
+        yarn clean && node scripts/build.js
+    )
+}
+
 pod_install() {
     [[ "$OSTYPE" == "darwin"* ]] \
         || return 0
@@ -31,6 +42,8 @@ jetify() {
 
 patch_package
 
+build_rn_packages
+
 pod_install
 
 jetify