cmd/go: include GOMODCACHE in documented list of environment variables

[dmitshur]

Go 1.15 adds the GOMODCACHE environment variable (#34527). It's mentioned in the release notes (CL 230537). Should it also be included in the list at https://tip.golang.org/cmd/go/#hdr-Environment_variables?

/cc @jayconrod @matloob @bcmills

[tpaschalis]

If it's included, would it go under General-purpose environment variables? Could I suggest something like

GOMODCACHE
	The directory where the go command will cache module downloads. 
	It defaults to GOPATH/pkg/mod.

[gopherbot]

Change https://golang.org/cl/241275 mentions this issue: cmd/go: include GOMODCACHE in 'go help environment'

[dmitshur]

There's a CL out to fix this (that just needs review), I think we don't want to release Go 1.15 without this, so marking release-blocker so this isn't missed by accident.