cmd/go: error if -modfile used with 'go install pkg@version'
'go install pkg@version' runs without a main module or a module root
directory. The -modfile flag cannot be used to set the module root
directory or to substitute a different go.mod file.
This error won't be reported if -modfile is set in
GOFLAGS. Unsupported flags there are generally ignored.
For #40276
Change-Id: I0b39b1fa9184c15c6e863b647d43c328710920f4
Reviewed-on: https://go-review.googlesource.com/c/go/+/258297
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com> Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>