put back the not-quite-real version in the nix packaging

default.nix

@@ -40,7 +40,15 @@ in
       tahoe-lafs = mach-nix.buildPythonPackage rec {
         inherit python providers;
         name = "tahoe-lafs";
-        version = "1.17.0";
+        # We add `.post999` here so that we don't accidentally *exactly* match
+        # the upstream Tahoe-LAFS version.  This avoids the misleading
+        # circumstance where the version in the Nix packaging *looks* like a
+        # real upstream Tahoe-LAFS revision but we have forgotten to update it
+        # so it is the *wrong* real upstream Tahoe-LAFS revision.  Hopefully
+        # the `.post999` looks weird enough that if someone really cares about
+        # the version in use they will notice it and go searching for what's
+        # going on and discover the real version specified by `src` below.
+        version = "1.17.0.post999";
         # See https://github.com/DavHau/mach-nix/issues/190
         requirementsExtra = ''
           pyrsistent < 0.17