cmd/go: in TestScript/mod_outside, run 'go build' before checking cmd/addr2line staleness

cmd/go: in TestScript/mod_outside, run 'go build' before checking cmd/addr2line staleness

If the build cache is cleaned (using 'go clean -cache' or similar), or
if a toolchain is freshly installed without warming the cache, the
build cache might not contain the dependencies needed to verify that
cmd/addr2line is not stale. In that case, the test should refill the
cache instead of failing.

Fixes #56889.

Change-Id: Ic6cf13b92bafa9c795e50eb0e4e1a9ae00ee8538
Reviewed-on: https://go-review.googlesource.com/c/go/+/452458
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>