Fix flutter shell script to handle properly shells that have CDPATH set (#17589)

Before this change, if CDPATH was set, and you type (from the flutter root) "bin/flutter --version" you would get an error.

This is because the cd command in follow_links was printing the directory Bash chose from the CDPATH to stdout, causing the path to have a newline in it, which understandably confuses the rest of the script.

bin/flutter

@@ -14,6 +14,8 @@
 
 set -e
 
+unset CDPATH
+
 function follow_links() {
   cd -P "${1%/*}"
   local file="$PWD/${1##*/}"