This moves default Firefox and WebKit checkouts to $HOME folder,
unless browser specific env variables are specified.
In other words:
- Firefox checkouts goes to `$HOME/firefox` unless there's a
`$FF_CHECKOUT_PATH` that specifies a custom location.
- WebKit checkout goes to `$HOME/webkit` unless there's a
`$WK_CHECKOUT_PATH` that specifies a custom location.
With this change, all build bots will now re-use checkouts
between builds, which should speed up compilation.
This way bash won't expand and post-process variable values in any way.
The changes are driven with
[`shellcheck`](https://github.com/koalaman/shellcheck)
