Avoid test failure when stubbing FreeBSD environment

[mislav]

I think Linux actions runner images now have a /usr/local/include/openssl/ssl.h file, which triggers this condition:

if is_freebsd && [ -f /usr/local/include/openssl/ssl.h ]; then
  # use openssl installed from Ports Collection
  package_option ruby configure --with-openssl-dir="/usr/local"
fi

We can't stub the root filesystem, so instead use RUBY_CONFIGURE_OPTS to override this in tests.

https://github.com/rbenv/ruby-build/actions/runs/5719870214/job/15517987944#step:4:18

/cc @hsbt #2227