Free a build node's disk after every build and refuse work under a floor of free space (issue 1834)

A full / made every check on shanks end 'store never became ready' with the reason erased by --rm.
Builds now remove their local image tag, narrowed source and context clone; a build or check
refuses under 20 GiB free on / and the workspace, saying so; the throwaway store is no longer
started with --rm and a store that does not come up reports its exit code and last 20 log lines.

internal/builder/build_source_test.go

@@ -86,11 +86,14 @@ func TestAnImageCompilingGoIsHandedItsBuildSourceAndSaysIt(t *testing.T) {
 	}
 	for file, want := range map[string]bool{"proxy/main.go": true, "internal/shared/s.go": true, "go.mod": true,
 		"cmd/ctl/main.go": false, "internal/only/o.go": false, "README.md": false} {
-		_, err := os.Stat(filepath.Join(workspace, "narrow-server", filepath.FromSlash(file)))
-		if (err == nil) != want {
-			t.Errorf("%s handed to the recipe: %v, wanted %v", file, err == nil, want)
+		held := slices.Contains(r.builtFrom, file)
+		if held != want {
+			t.Errorf("%s handed to the recipe: %v, wanted %v", file, held, want)
 		}
 	}
+	if _, err := os.Stat(filepath.Join(workspace, "narrow-server")); err == nil {
+		t.Error("the narrowed build source was left in the workspace")
+	}
 
 	// Said per repository: its own, the manifest and the recipe; the context's, the closure.
 	if len(got.Sources) != 2 {