cmd/go: in TestScript/mod_replace, download an explicit module path

As of CL 318629, 'go mod download' without arguments does not save
checksums for module source code. Without a checksum, 'go list' will
not report the location of the source code even if it is present, in
order to prevent accidental access of mismatched code.

Downloading an explicit module here also more clearly expresses the
intent of the test (“download this module and see where it is”), and
may be somewhat more efficient (since the test doesn't need source
code for the other modules in the build list).

Updates #45332

Change-Id: Ic589b22478e3ed140b95365bb6729101dd598ccc
Reviewed-on: https://go-review.googlesource.com/c/go/+/321956


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>