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.

cmd/mesh-builder/Dockerfile

@@ -16,7 +16,8 @@ RUN CGO_ENABLED=0 go build -trimpath -ldflags '-s -w' -o /mesh-builder ./cmd/mes
 FROM ${ALPINE_BASE}
 # git to clone what it is asked to build, and the docker client to build and push it. The daemon
 # is the machine's, reached through its socket — a build machine shares the runtime it was given
-# rather than running one inside itself.
-RUN apk add --no-cache git docker-cli
+# rather than running one inside itself. xz to read an upstream release an archive is pinned to
+# (novox/hq ADR 0316): Go's standard library reads gzip and bzip2 and not xz.
+RUN apk add --no-cache git docker-cli xz
 COPY --from=build /mesh-builder /usr/local/bin/mesh-builder
 ENTRYPOINT ["/usr/local/bin/mesh-builder"]