Build shell trace functionality lost in JDK-8076060

When running the build with LOG=trace, we (used to) have a neat feature that would print which prerequisites triggered each rule to run. This functionality was lost with JDK-8076060 where the SHELL variable setting this up was changed to be assigned using ':=' instead of ':'. I cannot see any reason this was done for this particular variable so assuming it was a mistake while changing other variables in the same file. Reverting it for SHELL fixes LOG=trace.