macOS system bash inadequate

Building wip/bup-git on macOS 15 (supported, 1 version back), configure
failed bacause /bin/bash, picked up as TOOLS+=bash, is version 3, and
the rest of the world has been on bash 4 and now 5 for a really long
time.

I therefore suggest commenting out the TOOLS_PLATFORM.bash in
mk/tools/Darwin.mk so pkgsrc bash is used.  Doing so made the bup build
work fine.

I know we're in careful mode now, and I don't want to just land this
without asking.  My guess is that it's a net win for stability and I
wouldn't be surprised if something that's broken now starts working.
It's also easy to revert.

So wiz@, whoever is running Q3, everyone else: WDYT?