main.star: add netbsd-arm to SLOW_HOSTS

As mentioned in https://go.dev/issue/63698#issuecomment-2450448780,
this might be a good starting point. Can be adjusted later as needed.

For golang/go#63698.

Change-Id: I851852981527e1dcdae18f535f16a37117791ce6
Reviewed-on: https://go-review.googlesource.com/c/build/+/624075
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Benny Siegert <bsiegert@gmail.com>
Auto-Submit: Benny Siegert <bsiegert@gmail.com>

main.star

@@ -340,6 +340,7 @@ SLOW_HOSTS = {
     "linux-ppc64le_power8": 2,
     "linux-ppc64le_power9": 2,
     "linux-riscv64": 2,
+    "netbsd-arm": 5,
     "netbsd-arm64": 2,
     "openbsd-amd64": 2,
     "openbsd-arm": 5,