build: Support proxy build arguments
requested to merge experimental/native-llb-I70534841d23d61e470cc2d8f5383f8bb02856655 into experimental/native-llb
This fixes a regression where support for HTTP_PROXY, HTTPS_PROXY,
FTP_PROXY, NO_PROXY, ALL_PROXY build arguments was lost due to the
LLB-only refactor.
Respect these build arguments by passing a respective llb.ProxyEnv run
option for all run related exec operations (target.RunEntrypoint,
target.Run, and target.RunShell).
Bug: T367922