test(ci): make android CI script more rerun-tolerant

if the ~/.npm-global directory already exists, then we don't need
to try (and maybe fail) to create it or re-install deps that are already
there

scripts/install.sh

@@ -1,8 +1,9 @@
 #!/bin/bash -e
 
-mkdir ~/.npm-global
 npm config set prefix '~/.npm-global'
 export PATH=~/.npm-global/bin:$PATH
-
-npm install -g lerna@3.22.1
-npm install -g react-native-cli >/dev/null 2>&1
+if [ -d ~/.npm-global ]; then
+ mkdir ~/.npm-global
+ npm install -g lerna@3.22.1
+ npm install -g react-native-cli >/dev/null 2>&1
+fi