ninja: refactor version,src

This patch fixes a problem with nix-prefetch (and thus a number of update
scripts) where it would throw the following error when run in nixpkgs:

```
error: attribute 'rev' missing
at pkgs/by-name/ni/ninja/package.nix:19:34:
   18|   pname = "ninja";
   19|   version = lib.removePrefix "v" finalAttrs.src.rev;
     |                                  ^
```

This seems to be caused by an oversight where nix-prefetch overrides
fetchFromGitHub in a weird way, where rev is not returned, but so far this
is the only package that is triggering this bug. The related issue in
nix-prefetch is still unhandled, hence this change.

See: msteen/nix-prefetch#54 #403032

pkgs/by-name/ni/ninja/package.nix

@@ -17,26 +17,25 @@
 
 stdenv.mkDerivation (finalAttrs: {
   pname = "ninja";
-  version = lib.removePrefix "v" finalAttrs.src.rev;
-
-  src =
+  version =
     {
-      # TODO: Remove Ninja 1.11 as soon as possible.
-      "1.11" = fetchFromGitHub {
-        owner = "ninja-build";
-        repo = "ninja";
-        rev = "v1.11.1";
-        hash = "sha256-LvV/Fi2ARXBkfyA1paCRmLUwCh/rTyz+tGMg2/qEepI=";
-      };
-
-      latest = fetchFromGitHub {
-        owner = "ninja-build";
-        repo = "ninja";
-        rev = "v1.12.1";
-        hash = "sha256-RT5u+TDvWxG5EVQEYj931EZyrHUSAqK73OKDAascAwA=";
-      };
+      "1.11" = "1.11.1";
+      latest = "1.12.1";
     }
-    .${ninjaRelease} or (throw "Unsupported Ninja release: ${ninjaRelease}");
+    .${ninjaRelease};
+
+  src = fetchFromGitHub {
+    owner = "ninja-build";
+    repo = "ninja";
+    rev = "v${finalAttrs.version}";
+    hash =
+      {
+        # TODO: Remove Ninja 1.11 as soon as possible.
+        "1.11" = "sha256-LvV/Fi2ARXBkfyA1paCRmLUwCh/rTyz+tGMg2/qEepI=";
+        latest = "sha256-RT5u+TDvWxG5EVQEYj931EZyrHUSAqK73OKDAascAwA=";
+      }
+      .${ninjaRelease} or (throw "Unsupported Ninja release: ${ninjaRelease}");
+  };
 
   depsBuildBuild = [ buildPackages.stdenv.cc ];