!359 started using IfCmd instead of Command to execute the cache-extractor and
cache-archiver commands. These are passed URLs that include various characters
that need escaping in the IfCmd context, but IfCmd was a naked strings.Join()
in all cases.

This commit refactors the Command() functions so they and IfCmd() use the same
code to generate the commands that are run.

PsWriter.IfCmd() is further refactored to avoid iex as I couldn't make it work
in my powershell installation at all.