The SAVE_DEV_BUNDLE_TARBALL environment variable is already supported in the Unix version and this mimics that functionality in the Windows script. Use of this flag makes debugging much less painful, especially when switching branches (for example, during a `git bisect`) as the dev_bundle can be cached locally.
I also replaced a couple un-related uses of `echo` with `Write-Host` which is the preferred way to output messages in PowerShell as it supports more options.
Fixes: developer experience
This duplicates logic we had for Mac/Linux that lets you
set a USE_TEST_DEV_BUNDLE_SERVER environment variable
to try running a newly built dev bundle before publishing
it.
