**Preamble**
1. We're trying to setup a windows-based github self-hosted runner in the
playwright-internal repo.
1. Commands on Windows are mandated to have total arguments length
less then 32767 characters.
1. On windows, github self-hosted runner framework puts repository
checkout at `c:\w\playwright-internal\playwright-internal`
1. Our scripts create a checkout at
`c:\w\playwright-internal\playwright-internal\browser_patches\firefox\checkout`
1. One of the scripts in Firefox buildsystem tries to execute a command,
passing lots of absolute paths to various webidl's
1. The command fails due to restriction in (2)
**Problem**
Firefox build fails since checkout is deeply nested and hits max arg
size on windows.
**Solution**
This patch introduces a new variable `FF_CHECKOUT_PATH` that is
respected by all browser-related scripts. This way we'll be able
to checkout firefox to `c:\firefox` and avoid hitting long arguments
limit.
This strictly defines Firefox build folder as `obj-build-playwright`.
Currently, Firefox build folder encodes current Mac OS version including
patch versions, and thus we might end up with multiple different build
folders.
