bash: use SSH_SOURCE_BASHRC (like Apple bash).

Apple's /bin/bash is built with SSH_SOURCE_BASHRC, which causes
~/.bashrc to get sourced when the shell is run non-interactively over or
sshd/rshd. Linux distros usually also build with SSH_SOURCE_BASHRC (at
least, Debian does). This makes Homebrew do the same.

Closes #23234.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>