cmd/go: print offending -mod value in workspace mode

Workspace mode only allows -mod to be set to 'readonly'. When returning
the error for other values of -mod, specify the the value in the error
so it's easier to see what's going on when the go command is invoked
through another tool.

For #45713, #48186

Change-Id: I3ba206a71c7a20c18aeccfa48f3c9dc935a180a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/347593


Trust: Michael Matloob <matloob@golang.org>
Trust: Daniel Martí <mvdan@mvdan.cc>
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Bryan C. Mills <bcmills@google.com>