libexpr: fix stack overflow in getDerivations on deeply nested structures

nix-instantiate on deeply nested structures with recurseForDerivations
(e.g., `let x = { recurseForDerivations = true; more = x; }; in x`)
caused an uncontrolled OS-level stack overflow with no Nix stack trace.

Fix by adding call depth tracking to getDerivations, integrating with
Nix's existing max-call-depth mechanism. Now produces a controlled
"stack overflow; max-call-depth exceeded" error with a proper stack
trace.

src/libexpr/get-drvs.cc

@@ -378,6 +378,8 @@ static void getDerivations(
     Done & done,
     bool ignoreAssertionFailures)
 {
+    auto _level = state.addCallDepth(vIn.determinePos(noPos));
+
     Value v;
     state.autoCallFunction(autoArgs, vIn, v);
 

tests/functional/simple.sh

@@ -35,3 +35,8 @@ if test "$outPath" != "/foo/xxiwa5zlaajv6xdjynf9yym9g319d6mn-big-derivation-attr
     echo "big-derivation-attr.nix hash appears broken, got $outPath. Memory corruption in large drv attr?"
     exit 1
 fi
+
+# Test that nix-instantiate on a deeply nested recurseForDerivations structure
+# produces a controlled stack overflow error rather than a segfault.
+expectStderr 1 nix-instantiate --expr 'let x = { recurseForDerivations = true; more = x; }; in x' \
+  | grepQuiet "stack overflow; max-call-depth exceeded"