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>

src/cmd/go/testdata/script/work.txt

@@ -4,7 +4,7 @@ go env GOWORK
 ! stdout .
 
 go work init ./a ./b
-cmp go.work go.work.want
+cmpenv go.work go.work.want
 go env GOWORK
 stdout '^'$WORK'(\\|/)gopath(\\|/)src(\\|/)go.work$'
 
@@ -69,7 +69,7 @@ use (
   ../src/a
 )
 -- go.work.want --
-go 1.18
+go $goversion
 
 use (
 	./a

src/cmd/go/testdata/script/work_edit.txt

@@ -1,10 +1,10 @@
 # Test editing go.work files.
 
 go work init m
-cmp go.work go.work.want_initial
+cmpenv go.work go.work.want_initial
 
 go work edit -use n
-cmp go.work go.work.want_use_n
+cmpenv go.work go.work.want_use_n
 
 go work edit -go 1.18
 cmp go.work go.work.want_go_118
@@ -39,11 +39,11 @@ module m
 
 go 1.18
 -- go.work.want_initial --
-go 1.18
+go $goversion
 
 use ./m
 -- go.work.want_use_n --
-go 1.18
+go $goversion
 
 use (
 	./m