When a Freestyle job performs a checkout to a subdirectory the containing build script needs to make guesses about the name of that directory. Make it easier for containing scripts by adding the GIT_CHECKOUT_DIR environment variable when a Freestyle job performs a checkout to a subdirectory.

Pipeline jobs and multibranch Pipeline jobs should not use the git plugin's checkout to a subdirectory facility. They should instead use the Pipeline steps which will perform the same type of operation within a Pipeline. For example, they should use the `dir` step or the `ws` step.