[release-branch.go1.15] cmd/go: use a real Go version in the go.mod files in TestScript/mod_readonly

For some reason, the go.mod file added to this test in CL 147281 lists
'go 1.20' instead of the version that was actually current when the
go.mod file was added.

That causes the test's behavior to change under lazy loading, because
1.20 is above the threshold to trigger lazy-loading invariants (1.17).

This backports CL 314049 to Go 1.15 in order to fix a spurious test
failure in a subsequent change.

For #46143
Updates #46142
Updates #36460

Change-Id: I92400996cb051ab30e99bfffafd91ff32a1e7087
Reviewed-on: https://go-review.googlesource.com/c/go/+/314049
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/319709
Reviewed-by: Jay Conrod <jayconrod@google.com>

Author: Bryan C. Mills

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

@@ -64,7 +64,7 @@ go list all
 -- go.mod --
 module m
 
-go 1.20
+go 1.15
 
 -- x.go --
 package x
@@ -79,7 +79,7 @@ require (
 -- go.mod.redundant --
 module m
 
-go 1.20
+go 1.15
 
 require (
 	rsc.io/quote v1.5.2
@@ -89,7 +89,7 @@ require (
 -- go.mod.indirect --
 module m
 
-go 1.20
+go 1.15
 
 require (
 	rsc.io/quote v1.5.2 // indirect