Set PKG_CONFIG_PATH when building ruby

The ruby openssl module build invokes `pkg-config --libs openssl` to
find libs to link against, e.g., `-lssl -lcrypto`. If the openssl dev
package is _not_ installed on the system, ruby still builds
successfully but skips the openssl module, so the Omnibus build fails
later on `bundle install` ("Could not load OpenSSL.")

If the openssl dev package _is_ installed on the system, it will use
those `.pc` files and everything will probably work, even though this
is technically incorrect.

So let's tell `pkg-config` to use the `.pc` files from the Omnibus
openssl component by setting `PKG_CONFIG_PATH` to

    #{install_dir}/embedded/lib/pkgconfig

Other Omnibus software definitions may need to set `PKG_CONFIG_PATH`,
too, but since I was able to build omnibus-chef-server successfully
with this patch I stopped here.