Fixes #1114: Quote command and arguments in ShellScript.GetFullCommand()

This commit updates ShellScript.GetFullCommand() to surround each command element with quotes so that command arguments that contain spaces are kept intact. This fixes an issue where executors that used SSH would fail to run properly after the shell auto-detection changes were made to shells/bash.go, as the argument containing the detection script would incorrectly interpreted as multiple arguments (other executors don't use this function, so they were not affected).