Run File::Find::find with no_chdir=>1

Without no_chdir=>1 the unshared child process in unshare mode needs
read permissions for the directory from which mmdebstrap is executed.
With this change, the current working directory does not need to be
world-readable anymore.

Closes: #1005857
Reported-by: Trent W. Buck <trentbuck@gmail.com>

README.md

@@ -154,3 +154,4 @@ Contributors
  - Konstantin Demin
  - Trent W. Buck
  - Vagrant Cascadian
+ - Gioele Barabucci

mmdebstrap

@@ -4094,7 +4094,10 @@ sub approx_disk_usage {
             $installed_size += 1;
         }
     };
-    find($scan_installed_size, $directory);
+    # We use no_chdir because otherwise the unshared user has to have read
+    # permissions for the current working directory when producing an ext2
+    # image. See https://bugs.debian.org/1005857
+    find({ wanted => $scan_installed_size, no_chdir => 1 }, $directory);
 
     # because the above is only a heuristic we add 10% extra for good measure
     return int($installed_size * 1.1);