cmd/go: make work and work_edit script tests version-independent

The work and work_edit script tests ran go work init, which put the
current Go version into the go.work files. Before this change, the tests
used cmp to compare the outputs with a file that contained a literal
"go 1.18" line. Instead, use cmpenv so we can compare with
"go $goversion". (Some of the test cases still compare against files
that contain "go 1.18" lines, but these tests explicitly set the version
to go 1.18 either in the original go.work files or using go work edit.)

Change-Id: Iea2caa7697b5fe5939070558b1664f70130095ce
Reviewed-on: https://go-review.googlesource.com/c/go/+/388514


Trust: Michael Matloob <matloob@golang.org>
Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Carlos Amedee <carlos@golang.org>