Prove move-out on a machine that is not Arch (issue 1024)

The merge check's build container has no pacman, so the acceptance test now stubs it and names the host it was built for, as other command tests do; nothing in the node-engine itself changes.

cmd/mesh-host/move_out_test.go

@@ -43,9 +43,16 @@ func TestAMoveOutDirectoryKeepsOnlyWhatItKnows(t *testing.T) {
 		t.Fatal(err)
 	}
 	t.Cleanup(apply.UseAccountsFileForTest(passwd))
-	was := builtFor
+	// Independent of the machine: an arch host with a pacman on PATH that answers and holds nothing, as
+	// TestReplay233 puts one there (it answers with an error); nothing here declares a package.
+	bin := t.TempDir()
+	if err := os.WriteFile(filepath.Join(bin, "pacman"), []byte("#!/bin/sh\nexit 0\n"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("PATH", bin+string(os.PathListSeparator)+os.Getenv("PATH"))
+	wasBuilt := builtFor
 	builtFor = "arch"
-	t.Cleanup(func() { builtFor = was })
+	t.Cleanup(func() { builtFor = wasBuilt })
 
 	stray := filepath.Join(dir, "id_stray")
 	if err := os.WriteFile(stray, []byte("somebody else's key"), 0o600); err != nil {