Introduce a variable to enable shell tracing on bash, cmd.exe and powershell.exe

## What does this MR do?

Allows users to enable debug tracing during builds, using `set -o xtrace`, `@echo on` and `Set-PSDebug -Trace 1`, respectively.

## Why was this MR needed?

Debugging failures partway through the execution of a generated script is currently tricky, as any one of a number of commands could have failed. Enabling this feature for a failing build will ease the process considerably.

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

Should the variable have a different name?

Should PSDebug be -Trace 1 or -Trace 2?

Should cmd.exe explicitly set `@echo on`? I'm pretty sure it's the default

## Does this MR meet the acceptance criteria?

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

## What are the relevant issue numbers?

Closes #1711

See merge request !339