main.star: add linux-amd64-longtest as a presubmit for pkgsite

This will run the tests that the linux-amd64_docker builder ran.
If this and the -screentest builder are healthy we can then proceed
to turn down the docker builders.

Also disable the pkgsite longtest/screentest builders for
1.26 or 1.27 since they will try to run the docker tests,
similar to how they're disabled for _docker. And take
linux-amd64-longtest off the known issue builder list because
the x_pkgsite gotip-linux-amd64-screentest has been healthy.

For golang/go#82561

Change-Id: Idfbf554ea98d57adcd31cf6d5f3a49996a6a6964
Reviewed-on: https://go-review.googlesource.com/c/build/+/856242
Reviewed-by: Ethan Lee <ethanalee@google.com>
Reviewed-by: Michael Matloob <matloob@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>