texlive: put snapshot date into the version

pkgs/tools/typesetting/tex/texlive/combine.nix

@@ -2,10 +2,12 @@ params: with params;
 # combine =
 args@{
   pkgFilter ? (pkg: pkg.tlType == "run" || pkg.tlType == "bin" || pkg.pname == "core")
-, extraName ? "combined", ...
+, extraName ? "combined"
+, extraVersion ? ""
+, ...
 }:
 let
-  pkgSet = removeAttrs args [ "pkgFilter" "extraName" ] // {
+  pkgSet = removeAttrs args [ "pkgFilter" "extraName" "extraVersion" ] // {
     # include a fake "core" package
     core.pkgs = [
       (bin.core.out // { pname = "core"; tlType = "bin"; })
@@ -37,7 +39,7 @@ let
     (map (p: p.outPath) (builtins.filter lib.isDerivation pkgs));
 
 in (buildEnv {
-  name = "texlive-${extraName}-${bin.texliveYear}";
+  name = "texlive-${extraName}-${bin.texliveYear}${extraVersion}";
 
   extraPrefix = "/share/texmf";
 

pkgs/tools/typesetting/tex/texlive/default.nix

@@ -88,6 +88,12 @@ let
         ++ combinePkgs (attrs.deps or {});
     };
 
+  snapshot = {
+    year = "2020";
+    month = "10";
+    day = "09";
+  };
+
   # create a derivation that contains an unpacked upstream TL package
   mkPkg = { pname, tlType, revision, version, sha512, postUnpack ? "", stripPrefix ? 1, ... }@args:
     let
@@ -110,7 +116,7 @@ let
         #"ftp://tug.org/texlive/historic/2019/tlnet-final/archive"
 
         # Daily snapshots hosted by one of the texlive release managers
-        https://texlive.info/tlnet-archive/2020/10/09/tlnet/archive
+        "https://texlive.info/tlnet-archive/${snapshot.year}/${snapshot.month}/${snapshot.day}/tlnet/archive"
       ];
 
       src = fetchurl { inherit urls sha512; };
@@ -174,6 +180,7 @@ in
           (combine {
             ${pname} = attrs;
             extraName = "combined" + lib.removePrefix "scheme" pname;
+            extraVersion = ".${snapshot.year}${snapshot.month}${snapshot.day}";
           })
         )
         { inherit (tl)