oo-exec-ruby: Set PATH &c. directly, not using scl

oo-exec-ruby: Set the PATH and LD_LIBRARY_PATH variables using straight
Bash code rather than using the scl command.

The OpenShift node platform munges PATH in some places, and a nested scl
invocation will not update the variables.  This fix is necessary so that
when we run oo-exec-ruby, munge PATH, and then run oo-exec-ruby, we get the
ruby193 scl back in the PATH.

This commit fixes bug 1017248.