lib.version: change pre-git to post-git on the release branch

When there is no .version-suffix file in nixpkgs (like when fetching
nixpkgs with builtins.fetchGit), lib.version suffixes the version string
with "pre-git". The "pre" bit is special cased in
builtins.compareVersions which means "20.03pre-git" is interpreted as
"less than 20.03". This is clearly wrong for the release-20.03 branch
*after* the release has been made.

Change the suffix to "post-git" to make code like this behave the same
whether nixpkgs is fetched from git or the channel (which has
.version-suffix file):

  lib.versionOlder lib.version "20.03"
  lib.versionAtLeast lib.version "20.03"

(Currently the result depend on how nixpkgs was obtained!)

This change should be made part of the release process.

lib/trivial.nix

@@ -178,7 +178,7 @@ rec {
     let suffixFile = ../.version-suffix;
     in if pathExists suffixFile
     then lib.strings.fileContents suffixFile
-    else "pre-git";
+    else "post-git";
 
   /* Attempts to return the the current revision of nixpkgs and
      returns the supplied default value otherwise.