Describe the bug
When running an external, $ is always evaluated as an env var, so it must always be escaped. This perhaps applies to other special characters for the shell under which we execute externals.

To Reproduce
Compare

$ ^echo yes | awk '{ print $0 }'

to

$ ^echo yes | awk '{ print \$0 }'
yes

Expected behavior
I would expect one of two possible behaviours:

1. Since nu doesn't do environment variables at the moment, perhaps escaping would be unnecessary.
2. Similar to other shells, "..." would interpolate env vars and '...' would not.

Configuration (please complete the following information):

• OS: OSX
• Version 10.15.3