cmd/buildlet/stage0: define env for host-linux-ppc64-sid
cmd/buildlet/stage0: define env for host-linux-ppc64-sid

This should be identical to host-linux-ppc64-osu.

Change-Id: I673e64469ee3dba92cb772c739776a558fb4ef49
Reviewed-on: https://go-review.googlesource.com/c/build/+/426914
Reviewed-by: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Reviewed-by: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>
Run-TryBot: Paul Murphy <2e1d8d6f81223fe22f04c5349bc2b02555a275af@ibm.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/build,golang/build,golang/build,golang/build