godoc: allow change of GOOS and GOARCH through environment

[Rozruch]

Currently there is no way to change the GOOS and GOARCH thorugh godoc.Presentation.Get[Pkg,Cmd]PageInfo() as the function trims the goos and goarch arguments passed to handlerServer.GetPageInfo.
When using this tool as a package, it is possible to change the target goos and goarch through environment variables only when the project is compiled into a binary.

My proposal is to check if the environment has those variables set (They are used when setting context.Default.[GOOS,GOARCH] anyway, but on package import).

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[dmitshur]

Thanks for sending a change.

Per https://golang.org/doc/contribute.html#design, please open an issue and fill in the template, then add a "Fixes golang/go#nnn." line here.

I'd like to understand this change better, and it's more appropriate to discuss it on the issue tracker.

Thanks. (Sorry about closing, clicked the wrong button.)