Fix and extend nix-shell baseDir test

src/libcmd/common-eval-args.cc

@@ -202,7 +202,7 @@ Bindings * MixEvalArgs::getAutoArgs(EvalState & state)
         auto v = state.allocValue();
         std::visit(overloaded {
             [&](const AutoArgExpr & arg) {
-                state.mkThunk_(*v, state.parseExprFromString(arg.expr, state.rootPath(".")));
+                state.mkThunk_(*v, state.parseExprFromString(arg.expr, true ? state.rootPath(absPath(getCommandBaseDir())) : state.rootPath(".")));
             },
             [&](const AutoArgString & arg) {
                 v->mkString(arg.s);

src/libutil/args/root.hh

@@ -29,6 +29,7 @@ struct Completions final : AddCompletions
  */
 class RootArgs : virtual public Args
 {
+protected:
     /**
      * @brief The command's "working directory", but only set when top level.
      *

src/nix-build/nix-build.cc

@@ -183,6 +183,9 @@ static void main_nix_build(int argc, char * * argv)
     struct MyArgs : LegacyArgs, MixEvalArgs
     {
         using LegacyArgs::LegacyArgs;
+        void setBaseDir(Path baseDir) {
+            commandBaseDir = baseDir;
+        }
     };
 
     MyArgs myArgs(myName, [&](Strings::iterator & arg, const Strings::iterator & end) {
@@ -290,6 +293,9 @@ static void main_nix_build(int argc, char * * argv)
     state->repair = myArgs.repair;
     if (myArgs.repair) buildMode = bmRepair;
 
+    if (inShebang) {
+        myArgs.setBaseDir(absPath(dirOf(script)));
+    }
     auto autoArgs = myArgs.getAutoArgs(*state);
 
     auto autoArgsWithInNixShell = autoArgs;