nix: most-basic test of postResholve

Should probably do better here, but I've burned more of the weekend
than I hoped already and want to get a resholve bump cut before the
end of the day.

Author: abathur

CHANGELOG.md

@@ -1,9 +1,10 @@
 # Changelog
 
-## unreleased
+## v0.10.7 (unreleased)
 - nix: track a number of changes upstream in nixpkgs, notably
     - fix a problem in resholve's override API
     - nixfmt
+    - ~adopt currently unmerged PR adding a postResholve feature
 - skip ~failing msmtp(q) parse tests to unblock CI
 - nix: update flake
 - nix: fix some cross issues

nixpkgs/test.nix

@@ -157,6 +157,9 @@ rec {
         inputs = [ ];
       };
     };
+    postResholve = ''
+      echo "not a load-bearing test, just prove we exist"
+    '';
   };
   # demonstrate that we could use resholve in larger build
   module3 = stdenv.mkDerivation {