Fixes Global Node Prefix Error (#31740)

Summary:
Some users have `node` installed globally which sets a `PREFIX` by default (so it knows where to put pkgs). We are looking for the "right" node on the next step anyway and if the user is using `nvm`, `PREFIX` breaks it.

closes #31181

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[General] [Fixed] - Ignores global npm prefix

Pull Request resolved: #31740

Test Plan:
`yarn ios` now works, even if there's a `usr/local/bin/npm`

<img width="493" alt="Screen Shot 2021-06-17 at 10 14 08 AM" src="https://user-images.githubusercontent.com/2659478/122413946-c2f57200-cf54-11eb-817c-bd3c07ac50bf.png">

Reviewed By: yungsters

Differential Revision: D31237363

Pulled By: charlesbdudley

fbshipit-source-id: 4ee9c04f8b8ab4e815bafbe2d02e589d621577b4

scripts/find-node.sh

@@ -6,6 +6,10 @@
 
 set -e
 
+# remove global prefix if it's already set
+# the running shell process will choose a node binary and a global package directory breaks version managers
+unset PREFIX
+
 # Support Homebrew on M1
 HOMEBREW_M1_BIN=/opt/homebrew/bin
 if [[ -d $HOMEBREW_M1_BIN && ! $PATH =~ $HOMEBREW_M1_BIN ]]; then