texlive: use versioned tarballs

Announced in [1], versioned tarballs allow to make sure that a
specific version is fetched. This does not guarantee that all previous
versions are retained on the main mirrors.

Logically, we would want to first try to download versioned tarballs
from any mirror and only then try the unversioned ones. But right now
we only have two mirrors and only some of the tarballs are versioned
in texlive-2019, so the order is changed to not hammer the weak
tug.org mirror.

[1] https://tug.org/pipermail/tex-live/2019-September/044086.html

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

@@ -89,7 +89,7 @@ let
     };
 
   # create a derivation that contains an unpacked upstream TL package
-  mkPkg = { pname, tlType, version, sha512, postUnpack ? "", stripPrefix ? 1, ... }@args:
+  mkPkg = { pname, tlType, revision, version, sha512, postUnpack ? "", stripPrefix ? 1, ... }@args:
     let
       # the basename used by upstream (without ".tar.xz" suffix)
       urlName = pname + lib.optionalString (tlType != "run") ".${tlType}";
@@ -97,8 +97,12 @@ let
       fixedHash = fixedHashes.${tlName} or null; # be graceful about missing hashes
 
       urls = args.urls or (if args ? url then [ args.url ] else
-              map (up: "${up}/${urlName}.tar.xz") urlPrefixes
-            );
+        lib.concatMap
+          (up: [
+            "${up}/${urlName}.r${toString revision}.tar.xz"
+            "${up}/${urlName}.tar.xz" # TODO To be removed for telive 2020
+          ])
+          urlPrefixes);
 
       # The tarballs on CTAN mirrors for the current release are constantly
       # receiving updates, so we can't use those directly. Stable snapshots