v12: Fix lazy sed for RRTMGP

[mathomp4]

As found by @michellefrazer, the sed in our runscripts was not quite good. This code:

   cat $instance.tmp | sed -e '/RRTMG/ s#RRTMG#RRTMGP#' > $instance

works great the first time around. But if you run multiple segments with the same scratch dir, well, you go from RRTMG to RRTMGP to RRTMGPP and boom, crash.

So we add word boundary markers to the sed to look only for the complete word RRTMG.

NOTE: The correct fix is to change the files in GOCART or do something smarter.