cmd: update vendored golang.org/x/mod

This pulls in golang.org/cl/250920 which rejects Windows shortnames as
path components in module.CheckImportPath (as is already done in
cmd/go/internal/get's copy of CheckImportPath). This will allow us to replace
the copy of CheckImportPath with the original.

This also pulls in golang.org/cl/250919 which rejects + in CheckPath and
CheckImportPath, and golang.org/cl/235597, which adds methods to the zip
package for gorelease, but shouldn't affect cmd.

This change also updates the cmd/go test case TestScript/mod_bad_filenames
to reflect that golang.org/x/mod/zip error messages now include filenames
for bad file names that can't be included in zip archives.

Updates #29101

Change-Id: I7f654325dc33b19bc9c6f77a56546747add5a47f
Reviewed-on: https://go-review.googlesource.com/c/go/+/251877


Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>