x/build: add macOS security trybots to LUCI

[dmitshur]

We have macOS builders in LUCI, but they're not yet available as security trybots. This is the tracking issue for that.

[gopherbot]

Change https://go.dev/cl/582095 mentions this issue: main.star: add security-only known issues

[gopherbot]

Change https://go.dev/cl/582096 mentions this issue: main.star: add darwin-amd64_14 builder to the internal host

[gopherbot]

Change https://go.dev/cl/582036 mentions this issue: cmd/makemac: add first security macOS trybot

[gopherbot]

Change https://go.dev/cl/582558 mentions this issue: main.star: permit makemac to list security bots