(PUP-3244) Add a hashbang line to the acceptance test enc

Ruby's Kernel.exec has different behavior on different platforms when
handling a script without a hashbang line. On rhel6, for instance:

[root@a3m9lej0pifviyl ~]# cat /root/test
echo 'foo'
[root@a3m9lej0pifviyl ~]# /root/test arg
foo
[root@a3m9lej0pifviyl ~]# irb
irb(main):001:0> exec('/root/test', 'arg')
Errno::ENOEXEC: Exec format error - /root/test

from (irb):1:in `exec'
from (irb):1
from :0

This seems to depend on how the underlying platforms exec functions
handle this case. Rewriting the enc as a ruby file with a hashbang line
sidesteps this issue and allows the test to run. Also dropped the
environment.conf usage since it seemed superfluous.