design: add 40276-go-install.md

Draft design for 'go install' functionality to install executables in
module mode outside a module at a specific version.

For golang/go#30515
For golang/go#40276

Change-Id: Iaec50eeae92148be51e6acb1d3a9488bb2587bc8
Reviewed-on: https://go-review.googlesource.com/c/proposal/+/243077
Trust: Jay Conrod <[email protected]>
Reviewed-by: Jay Conrod <[email protected]>