Improve verbose logging of the .pipe() method

[ehmicky]

The following:

await $`cat package.json`
  .pipe`head -n2`

Is currently logged like:

[19:49:00.360] cat package.json
[19:49:00.364] head -n

This does not show well that those processes are piped. Instead, we should log those as:

[19:49:00.360] $ cat package.json
[19:49:00.364] | head -n

Adding a prefix would also allow to better differentiate commands from their outputs (which we will add with #751).