niv candid: update f7269bc2 -> 81c43380 (#4920)

## Changelog for candid:
Branch: master
Commits: [dfinity/candid@f7269bc2...81c43380](dfinity/candid@f7269bc...81c4338)

* [`81c43380`](dfinity/candid@81c4338) fix: label hash fix ([dfinity/candid⁠#597](https://togithub.com/dfinity/candid/issues/597))

Author: dfinity-bot

nix/sources.json

@@ -6,10 +6,10 @@
         "homepage": "",
         "owner": "dfinity",
         "repo": "candid",
-        "rev": "f7269bc25acf1691411ff91840efe0247ddf6d53",
-        "sha256": "1bs5jbljjwr9g8qz6crfr5iicmlrary28lhkziqcm1cxz15kb6r4",
+        "rev": "81c433800c22c37d30285286fdb930baf5ee3727",
+        "sha256": "01cl4gclnv9sqcjn4sxjwgza26skmkzfh61k1j5bnhwlb4z31lzf",
         "type": "tarball",
-        "url": "https://github.com/dfinity/candid/archive/f7269bc25acf1691411ff91840efe0247ddf6d53.tar.gz",
+        "url": "https://github.com/dfinity/candid/archive/81c433800c22c37d30285286fdb930baf5ee3727.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "esm": {