fix(flake): set the real 2.1.0 srcHash

The flake shipped with srcHash = lib.fakeHash, so consuming it with
inputs.qbz.url = "github:vicrodh/qbz" failed to evaluate. Set the
published v2.1.0 tag hash (verified with nix-prefetch-url --unpack).

Closes #744.

cc

flake.nix

@@ -19,7 +19,7 @@
         qbzVersion = "2.1.0";
         qbzRev     = "v${qbzVersion}";
         # POST-RELEASE: replace from the published v2.1.0 tag.
-        srcHash    = pkgs.lib.fakeHash;
+        srcHash    = "sha256-Yc5f7DjAFAjYgTnT2yv7zjIxmXPJ8ZdLaoPnHJoFRX0=";
 
         # Libraries opened by name at runtime rather than linked into qbz.
         # Qt's own graphics/plugin closure is handled by wrapQtAppsHook.