Keep TestReplay867 off the build node's real free space (issue 1834, note 12)

The check's floor of free space reads the disk the test runs on, so
every mesh-controller check failed while its build node had under
20 GiB free, whatever the change (seen in the try of issue 1773's fix).
The replay stubs the free space, as internal/builder's tests do.

cmd/mesh-controller/replay867_test.go

@@ -46,6 +46,11 @@ func replay867(t *testing.T, recheck bool) {
 	if _, err := exec.LookPath("go"); err != nil {
 		t.Skip("no Go toolchain on this machine to build a judge with")
 	}
+	// The check's floor of free space (novox/hq issue 1834) reads the disk the test runs on, which is none of this
+	// replay's business: a build node under the floor failed every mesh-controller check here (issue 1834, note 12).
+	wasFree := builder.FreeBytes
+	builder.FreeBytes = func(string) (uint64, error) { return 500 << 30, nil }
+	t.Cleanup(func() { builder.FreeBytes = wasFree })
 	r := theGroupOfIssue867(t)
 	for _, c := range []struct {
 		what, repo string