cmd/link: don't let dsymutil delete our temp directory
To work around #59026, where dsymutil may not clean up its temp
directory at exit, we set DSYMUTIL_REPRODUCER_PATH to our temp
directory so it uses that, and we can delete it at the end.
In Xcode 16 beta, dsymutil deletes the DSYMUTIL_REPRODUCER_PATH
directory even if it is not empty. We still need our tmpdir at the
point, so give a subdirectory to dsymutil instead.
For #68088.
Change-Id: I18759cc39512819bbd0511793ce917eae72245d6
Reviewed-on: https://go-review.googlesource.com/c/go/+/593659 Reviewed-by: Than McIntosh <thanm@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>