Our bash shell integration code uses ENV (in POSIX mode) to bootstrap
our shell integration script. This had the side effect of overwriting an
existing ENV value.
This change preserves ENV by storing it temporarily in GHOSTTY_BASH_ENV.
Note that this doesn't enable --posix mode support for automatic shell
integration. (--posix does work; we just skip shell integration when
that flag is specified.) We can reconsider implementing full --posix
support separately.