community/gnome-session: force bash login shell on workaround

See https://bugzilla.gnome.org/show_bug.cgi?id=736660 for context (you can probably scroll to the end of the discussion)

In alpine, we compile bash as-is, without "NON_INTERACTIVE_LOGIN_SHELLS", which is what makes it work in other distros. Forcing the non-POSIX compliant variation when we know we're bash should actually work.

Fixes #15396 (closed)