x/build: include all first-class ports in TryBot coverage for x repos against previous Go releases

[bcmills]

Go version

a95136a

Output of go env in your module/workspace:

N/A

What did you do?

Observe #65223.

What did you see happen?

Test failures for the x/sys repo on release-branch.go1.20 on windows as of https://go.dev/cl/556895, which had a LUCI-TryBot-Result +1 vote.

What did you expect to see?

Since https://go.dev/cl/556895 had a passing TryBot run, it should not have introduced failures on any supported release branch on any first class port.

[dmitshur]

Issue #17626 was having golang.org/x trybots have any kind of coverage for previous supported Go releases.
The linux/amd64 builder was chosen as it was highly available and least expensive, and was a significant improvement compared to no coverage at all. But, it indeed is limited and doesn't catch problems that affect other OSes only on older Go versions.

It would certainly be good if we're able to expand this to cover more ports, and since we already consider breakages in first-class ports to block releases, adding first-class port trybot coverage makes a good next target. Adding FeatureRequest label since this is an enhancement.

[dmitshur]

CL 568197 added previous Go release coverage for 3 more first-class ports: linux/386, darwin/amd64, and windows/amd64.