fix debugger crashing while printing envs

fixes #9932

tests/functional/debugger.sh

@@ -0,0 +1,13 @@
+source common.sh
+
+clearStore
+
+# regression #9932
+echo ":env" | expect 1 nix eval --debugger --expr '(_: throw "oh snap") 42'
+echo ":env" | expect 1 nix eval --debugger --expr '
+  let x.a = 1; in
+  with x;
+  (_: builtins.seq x.a (throw "oh snap")) x.a
+' >debugger-test-out
+grep -P 'with: .*a' debugger-test-out
+grep -P 'static: .*x' debugger-test-out

tests/functional/local.mk

@@ -127,7 +127,8 @@ nix_tests = \
   toString-path.sh \
   read-only-store.sh \
   nested-sandboxing.sh \
-  impure-env.sh
+  impure-env.sh \
+  debugger.sh
 
 ifeq ($(HAVE_LIBCPUID), 1)
   nix_tests += compute-levels.sh