go: $PWD interpolation and actual pwd of command in go:generate differ

[jmhodges]

My go version is go1.6.2 darwin/amd64

If you reference $PWD in a go:generate comment, it will be the directory the user ran go generate in, instead of the directory the go:generate comment.

It makes some workarounds for getting some prickly-about-paths commands harder to get right without making some assumptions about where go generate will be run.

[minux]

Working as documented. cmd/go replaces $PWD (and other environment variables) according to its environment, and so it will be replaced with the directory that go generate is run. Why not just use . to pass the package path to a command?

[jmhodges]

This was for use with protoc which has its own, different rules about how you import other .proto files and I was trying to reconcile them. Using relative paths and the such give it a hard time.

[bcmills]

This was fixed as #43862.