Work-around for go >= 1.16

[ggwpez]

Problem

The installation instructions for go-mobile do not work anymore with the specified minimum version of 1.16:

go install golang.org/x/mobile/cmd/gomobile@latest
gomobile init

This line seems to be responsible for the error:

go: downloading golang.org/x/mobile v0.0.0-20211103151657-e68c98865fb2
go: downloading golang.org/x/mod v0.4.2
go: downloading golang.org/x/tools v0.1.8-0.20211022200916-316ba0b74098
go: downloading golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1
go: downloading golang.org/x/sys v0.0.0-20210809222454-d867a43fc93e
gomobile: go install golang.org/x/mobile/cmd/gobind failed: exit status 1
go install: version is required when current directory is not in a module
	Try 'go install golang.org/x/mobile/cmd/gobind@latest' to install the latest version

My current work-around is to run:

go mod init example.com/m
go mod tidy
GO111MODULE=on  go get golang.org/x/mobile/cmd/...
GO111MODULE=on  go mod download golang.org/x/exp
GO111MODULE=off go get -d golang.org/x/mobile/cmd/gobind
gomobile init 

And then go mod download golang.org/x/mobile in the docker image before building the bindings.
However, this does not work anymore in go 1.17 🤡.

We can hopefully remove this when golang/go#46943 is fixed.
I will not open a MR on x/mobile because of the tedious dev process for external contributors.