python3Packages.msgspec: fix src hash

The source hash changes because they use `$Format:%d$` in their code,
and GitHub can substitute this with different things in different times,
see https://github.com/NixOS/nixpkgs/issues/84312.

In this case, the diff looks like this:

```
diff -ur /nix/store/v2yfg05q83i154bdi2s96g21zi079rgi-source/msgspec/_version.py /nix/store/94zddrfms1ikx61i0plxnqdd3gavfdzr-source/msgspec/_version.py
--- /nix/store/v2yfg05q83i154bdi2s96g21zi079rgi-source/msgspec/_version.py      1970-01-01 00:00:01.000000000 +0000
+++ /nix/store/94zddrfms1ikx61i0plxnqdd3gavfdzr-source/msgspec/_version.py      1970-01-01 00:00:01.000000000 +0000
@@ -24,7 +24,7 @@
     # setup.py/versioneer.py will grep for the variable names, so they must
     # each be defined on a line of their own. _version.py will just call
     # get_keywords().
-    git_refnames = " (HEAD -> main, tag: 0.19.0)"
+    git_refnames = " (tag: 0.19.0)"
     git_full = "dd965dce22e5278d4935bea923441ecde31b5325"
     git_date = "2024-12-27 11:06:58 -0600"
     keywords = {"refnames": git_refnames, "full": git_full, "date": git_date}
```

Fix the hash and add a comment explaining the situation.

(cherry picked from commit e72eda8dc7)

pkgs/development/python-modules/msgspec/default.nix

@@ -17,7 +17,11 @@ buildPythonPackage rec {
     owner = "jcrist";
     repo = "msgspec";
     tag = version;
-    hash = "sha256-g2yhw9fMucBHlGx9kAMQL87znXlQT9KbxQ/QcmUetqI=";
+    # Note that this hash changes after some time after release because they
+    # use `$Format:%d$` in msgspec/_version.py, and GitHub produces different
+    # tarballs depending on whether tagged commit is the last commit, see
+    # https://github.com/NixOS/nixpkgs/issues/84312
+    hash = "sha256-CajdPNAkssriY/sie5gR+4k31b3Wd7WzqcsFmrlSoPY=";
   };
 
   build-system = [ setuptools ];