refactor(cli): improve dump's temporary file handling (#7350)

- Create temporary directory inside of a temporary directory (useful for a work-in-progress Landlock implementation, as we will not want to "whitelist" the entirety of the /tmp directory in our case, i.e. /tmp/forgejo-dump-133552095).
- The database is always removed after dump is complete.
- The temporary directory is removed if no temporary directory has been explicitly set (as in, created by Forgejo in /tmp or equivalent).

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7350
Reviewed-by: Gusted <[e-mail]>
Co-authored-by: Panagiotis "Ivory" Vasilopoulos <[e-mail]>
Co-committed-by: Panagiotis "Ivory" Vasilopoulos <[e-mail]>

cmd/dump.go

···
 		&cli.StringFlag{
 			Name:    "tempdir",
 			Aliases: []string{"t"},
-			Value:   os.TempDir(),
 			Usage:   "Temporary dir path",
 		},
 		&cli.StringFlag{
···
 	}
 
 	tmpDir := ctx.String("tempdir")
+	if tmpDir == "" {
+		tmpDir, err = os.MkdirTemp("", "forgejo-dump-*")
+		if err != nil {
+			fatal("Failed to create temporary directory: %v", err)
+		}
+
+		defer func() {
+			if err := util.Remove(tmpDir); err != nil {
+				log.Warn("Failed to remove temporary directory: %s: Error: %v", tmpDir, err)
+			}
+		}()
+	}
+
 	if _, err := os.Stat(tmpDir); os.IsNotExist(err) {
 		fatal("Path does not exist: %s", tmpDir)
 	}
 
 	dbDump, err := os.CreateTemp(tmpDir, "forgejo-db.sql")
 	if err != nil {
-		fatal("Failed to create tmp file: %v", err)
+		fatal("Failed to create temporary file: %v", err)
 	}
 	defer func() {
 		_ = dbDump.Close()
 		if err := util.Remove(dbDump.Name()); err != nil {
-			log.Warn("Failed to remove temporary file: %s: Error: %v", dbDump.Name(), err)
+			log.Warn("Failed to remove temporary database file: %s: Error: %v", dbDump.Name(), err)
 		}
 	}()