static-nix-shell: add a srcRoot argument which allows more precisely specifying the source files and avoiding unnecessary rebuilds

pkgs/additional/static-nix-shell/default.nix

@@ -37,10 +37,19 @@ in rec {
     pkgsEnv,
     pkgExprs,
     srcPath ? pname,
+    srcRoot ? null,
     extraMakeWrapperArgs ? [],
     ...
   }@attrs:
   let
+    extraDerivArgs = lib.optionalAttrs (srcRoot != null) {
+      # use can use `srcRoot` instead of `src` in most scenarios, to avoid include the entire directory containing their
+      # source file in the closure, but *just* the source file itself.
+      # `lib.fileset` docs: <https://ryantm.github.io/nixpkgs/functions/library/fileset/>
+      src = lib.fileset.toSource {
+        root = srcRoot; fileset = lib.path.append srcRoot srcPath;
+      };
+    };
     pkgsStr = concatStringsSep "" (map
       (pname: " -p ${pname}")
       pkgExprs
@@ -79,7 +88,7 @@ in rec {
 
         runHook postInstall
       '';
-    } // (removeAttrs attrs [ "extraMakeWrapperArgs" "interpreter" "interpreterName" "pkgsEnv" "pkgExprs" "srcPath" ])
+    } // extraDerivArgs // (removeAttrs attrs [ "extraMakeWrapperArgs" "interpreter" "interpreterName" "pkgsEnv" "pkgExprs" "srcPath" ])
   );
 
   # `mkShell` specialization for `nix-shell -i bash` scripts.