[docs] Add information on virtualenv (#16670)

Many of the subcommands of the WPT CLI require the `virtualenv` utility,
but the installation instructions did not instruct contributors to
install it. Add an installation step describing how to install the
dependency.

docs/running-tests/from-local-system.md

@@ -11,6 +11,21 @@ On Windows, be sure to add the Python directory (`c:\python2x`, by default) to
 your `%Path%` [Environment Variable](http://www.computerhope.com/issues/ch000549.htm),
 and read the [Windows Notes](#windows-notes) section below.
 
+<!--
+  There does not appear to be a cross-platform means of installing `pip`.
+  https://github.com/web-platform-tests/wpt/pull/16670
+-->
+
+Install `pip`. On many systems, this can be achieved with the command `python
+-m ensurepip`. If this is not possible, use your system's package manager to
+install the `python-pip` package.
+
+Next, install `virtualenv` using the following command:
+
+```bash
+pip install virtualenv
+```
+
 To get the tests running, you need to set up the test domains in your
 [`hosts` file](http://en.wikipedia.org/wiki/Hosts_%28file%29%23Location_in_the_file_system).