oflow: use built-in CommandContext
oflow: use built-in CommandContext

ExecCommandPipe emulated the behaviour of CommandContext that was
not available in go1.6.  Use the maintained built-in version as the go minimum
version is 1.8.

safchain/skydive,skydive-project/skydive,lebauce/skydive,skydive-project/skydive,safchain/skydive,skydive-project/skydive,safchain/skydive,safchain/skydive,safchain/skydive,lebauce/skydive,lebauce/skydive,lebauce/skydive,lebauce/skydive,safchain/skydive,lebauce/skydive,skydive-project/skydive,skydive-project/skydive,lebauce/skydive,safchain/skydive,skydive-project/skydive,skydive-project/skydive,lebauce/skydive,safchain/skydive