dashboard: set GO_EXTLINK_ENABLED=1 for Alpine builder for now

Updates golang/go#18243

Change-Id: I76989c3f6b592fb2b68df86c853896f7a2ff7a25
Reviewed-on: https://go-review.googlesource.com/41798
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
1 file changed