Automatic merge from submit-queue
build: clean platform envs to prevent cross-contamination
**What this PR does / why we need it**: As I described in https://github.com/kubernetes/kubernetes/issues/37079#issuecomment-263733509, we are leaking platform compilation envs between build stages for different platforms in the non-parallel dockerized cross build. This PR uses a subshell for the non-parallel build, more closely matching the parallel build.
This also adds some logging, which had it existed previously, may have made the bug more immediately obvious.
**Which issue this PR fixes**: fixes#37079
cc @sebgoa @iTagir @saad-ali
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