Let an archive be an upstream release pinned by URL and sha256, mirrored by the build seat (hq issue 345, ADR 0316)

A mail client's release is about 80 MB per version, too large to keep in a
repository at every upgrade, and an add-on whose licence forbids copying it
to third parties may not be in a repository at all. An archive artifact may
now name url, sha256 and version (and file or within) instead of from: the
build seat fetches it once, refuses it when its digest is not the pin's and
names both, keeps it in its workspace under the digest (a vendor that serves
only its newest version cannot break a later build of the same pin), and
packs it like any archive, so a node gets the mesh's own archive by digest
and never asks the vendor. The build seat's image carries xz, which Go's
standard library does not read. The installation check takes the vendor's
address and version of such a pin for the world's, as a registry is an
image's.

internal/builder/builder.go

@@ -971,8 +971,19 @@ func one(ctx context.Context, run Runner, publish Publisher,
 		return catalogue.Built{Name: a.Name, Kind: a.Kind, Reference: reference}, nil
 
 	case catalogue.ArtifactArchive:
-		src.ownHas(cleanEntry(a.From) + "/**")
-		body, err := pack(filepath.Join(tree, a.From))
+		var body []byte
+		var err error
+		if a.URL != "" {
+			// An upstream file pinned by URL and sha256 (novox/hq ADR 0316): its build source is the pin, which
+			// is in module.json, always part of it.
+			body, err = upstreamArchive(ctx, workspace, a, say)
+			if err != nil {
+				return catalogue.Built{}, fmt.Errorf("%s: %s: %w", module, a.Name, err)
+			}
+		} else {
+			src.ownHas(cleanEntry(a.From) + "/**")
+			body, err = pack(filepath.Join(tree, a.From))
+		}
 		if err != nil {
 			return catalogue.Built{}, fmt.Errorf("%s: packing %s failed: %w", module, a.Name, err)
 		}