run: don't double-encode newlines

Instead of double-ecnoding newlines (`\n` -> `\\\n`), just leave them as is.

Without this change, multiline commands are executed as if they are one line.

An invocation like this:
```ruby
run <<-CMD
  echo
  hostname
CMD
```

previously echoed "hostname" instead of running the `hostname` command because it's being interpreted like: `echo hostname`  instead of `echo \n hostname`.