Fix ruby_home in msgpack test

Author: eregon

test/truffle/cexts/msgpack/msgpack.sh

@@ -4,7 +4,7 @@ source test/truffle/common.sh.inc
 
 jt gem-test-pack
 
-ruby_home="$PWD"
+ruby_home="$(jt ruby -e 'puts Truffle::Boot.ruby_home')"
 export PATH="$ruby_home/bin:$PATH"
 
 gem install "$(jt gem-test-pack)/gem-cache/bundler-1.16.5.gem" --local --no-document