[proposal] Add ability to copy presentations & credentials to clipboard for Linux CLI

**Is your feature request related to a problem? Please describe.**
The CLI now includes a way to copy the text contents of credentials & presentations to the clipboard, but it is not implemented for Linux.

**Describe the solution you'd like**
Make that copy-to-clipboard function work for Linux, too.

**Describe alternatives you've considered**
One alternative is status-quo without that functionality, but that's just mean.

**Additional context**
[Here is the 'if' statement that handles Mac & Windows](https://github.com/uport-project/veramo/blob/d3b87f52f2707253449867d3a872ec60aa3fbc35/packages/cli/src/explore/utils.ts#L24).