dashboard: skip vulndb on aix-ppc64

vulndb currently uses a dependency which does not build cleanly on
this os/arch combo. Until that issue is resolved, skip (this code is
not intended to be run on a large variety of systems, so it should be
fine for the foreseeable future.)

Fixes golang/go#49218

Change-Id: I174daf963dbb4493141330bae55398e9f97f0fc1
Reviewed-on: https://go-review.googlesource.com/c/build/+/360414
Trust: Roland Shoemaker <roland@golang.org>
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Roland Shoemaker <roland@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

dashboard/builders.go

@@ -2508,6 +2508,12 @@ func init() {
 				// (https://golang.org/issue/31567).
 				// Skip affected repos until the builder is fixed.
 				return false
+			case "vulndb", "vuln":
+				// vulndb currently uses a dependency which does not build cleanly
+				// on aix-ppc64. Until that issue is resolved, skip vulndb on
+				// this builder.
+				// (https://golang.org/issue/49218).
+				return false
 			}
 			return buildRepoByDefault(repo)
 		},