shells

Fix {Bash,Cmd,Ps}Writer.IfCmd to escape its arguments

## What does this MR do?

Refactors the shell writers to use the same escaping logic in their `Command()` and `IfCmd()` methods

## Why was this MR needed?

!359 started using `IfCmd` with URLs as arguments. URLs include various characters that need escaping, especially `&` in bash.

## Are there points in the code the reviewer needs to double check?

The powershell and cmd changes look sane, but I haven't managed to run the executors/shell tests. Although I don't think they exercise the cache path anyway.

## Does this MR meet the acceptance criteria?

- [ ] Documentation created/updated
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)

## What are the relevant issue numbers?

Closes #1820 

/cc @ayufan @tmaczukin @stanhu

See merge request !364