Backport os.Expand from Go v1.10.8

This is needed because Go v1.11+ changed the behavior in https://github.com/golang/go/commit/7bf631e1fc55839a6bff404a91597326e1addc10#diff-b30ffd1cf7612ccebac97fced83d5106 so that $ isn't gobbled unless the value is a valid shell variable (e.g. $FOOBAR, ${FOOBAR}).

Since this may break CI builds that rely on this legacy behavior, keep it for now.