cmd/go/internal/auth: fix .netrc lookup for URLs with specified port

The .netrc spec [1] defines credentials based on "machine name", so remove specified ports
from URL before looking for a match.

[1] https://www.gnu.org/software/inetutils/manual/html_node/The-_002enetrc-file.html

Fixes #37130

Change-Id: Iab993afba26c927454d6166111ad1e1a53dbce43
Reviewed-on: https://go-review.googlesource.com/c/go/+/218418


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>