makefile: Clean build-env before get-orig-source target

(cherry picked from commit 37211def6304d9bb692da89451650a094b793b16)
[makefile] Fix ifmap-python-client target (get-orig-source missing)

Thanks Ted!

(cherry picked from commit 480e92df570e19c6c1bfb80f5f10f10045a7f55d)