Allow arbitrary location for third-party iOS dependency cache

Summary:
RNTester currently has four third party dependencies that are fetched from the network at build time:

- glog
- double-conversion
- boost
- folly

These dependencies are cached to ~/.rncache by default. This location may not be ideal for use in CI, therefore the cache location is now made configurable via a RN_CACHE_DIR envvar.

Reviewed By: cpojer

Differential Revision: D15141391

fbshipit-source-id: b51d749412c49500a657bd18e4c9520ddb30ea2c

Author: hramos

scripts/ios-install-third-party.sh

@@ -4,7 +4,9 @@
 # This source code is licensed under the MIT license found in the
 # LICENSE file in the root directory of this source tree.
 
-if [ -d "$HOME/.rncache" ]; then
+if [ -n "$RN_CACHE_DIR" ] && [ -d "$RN_CACHE_DIR" ]; then
+  cachedir="$RN_CACHE_DIR"
+elif [ -d "$HOME/.rncache" ]; then
   cachedir="$HOME/.rncache" # react-native 0.57.8 and older
 else
   cachedir="$HOME/Library/Caches/com.facebook.ReactNativeBuild"