Make base_ubuntu_warden_rosetta host-architecture agnostic so it can run in CI

bosh-stemcell/spec/bosh/stemcell/stage_collection_spec.rb

@@ -330,6 +330,21 @@ module Bosh::Stemcell
           expect(stage_collection.build_stemcell_image_stages).to eq(build_stemcell_image_stages)
           expect(stage_collection.package_stemcell_stages("files")).to eq(package_stemcell_stages)
         end
+
+        it "does not include the rosetta stage" do
+          expect(stage_collection.build_stemcell_image_stages).to_not include(:base_ubuntu_warden_rosetta)
+        end
+
+        context "with the rosetta variant" do
+          let(:operating_system) { OperatingSystem.for("ubuntu", "resolute-rosetta") }
+
+          # The stage has to run before bosh_clean/bosh_harden, which is why its
+          # position rather than its mere presence is asserted.
+          it "inserts the rosetta stage directly after base_warden" do
+            stages = stage_collection.build_stemcell_image_stages
+            expect(stages[stages.index(:base_warden) + 1]).to eq(:base_ubuntu_warden_rosetta)
+          end
+        end
       end
     end
   end

bosh-stemcell/spec/stemcells/rosetta_spec.rb

@@ -70,7 +70,12 @@
     end
 
     # The architecture check above would still pass if the binary could not
-    # actually run, so exercise a real round-trip.
+    # actually run, so exercise a real round-trip — but only on a host whose
+    # kernel can run arm64. An x86-64 CI worker has no arm64 handler and every
+    # exec fails with "Exec format error"; there the exec path is covered
+    # instead by the manual build on Apple Silicon
+    # (docs/ci-warden-rosetta-build.md). Every other assertion in this file is
+    # static and runs everywhere.
     describe command(
       "set -e; " \
       "rm -rf /tmp/tar-spec; mkdir -p /tmp/tar-spec/src /tmp/tar-spec/out; " \
@@ -80,7 +85,14 @@
       "grep -q payload /tmp/tar-spec/out/src/probe; " \
       "rm -rf /tmp/tar-spec"
     ) do
-      it("extracts an archive it just created") { expect(subject.exit_status).to eq(0) }
+      it "extracts an archive it just created" do
+        if command("/lib/ld-linux-aarch64.so.1 --help").exit_status != 0
+          skip("this build host cannot execute arm64 binaries; the tar " \
+               "round-trip is the only check skipped, all static assertions ran")
+        end
+
+        expect(subject.exit_status).to eq(0)
+      end
     end
   end