[substitutions] fix #10825 set evaluation error (#10830)

tests/unit_tests/fixtures/substitutions/00-simple_var.input.yaml

@@ -8,6 +8,9 @@ substitutions:
   var21: "79"
   value: 33
   values: 44
+  position:
+    x: 79
+    y: 82
 
 test_list:
   - "$var1"
@@ -27,3 +30,7 @@ test_list:
   - !literal Literal $values ${are not substituted}
   - !literal ["list $value", "${is not}", "${substituted}"]
   - !literal {"$dictionary": "$value", "${is not}": "${substituted}"}
+  - |- # Test parsing things that look like a python set of sets when rendered:
+    {{{ "x", "${ position.x }"}, { "y", "${ position.y }"}}}
+  - ${ '{{{"AA"}}}' }
+  - ${ '"HELLO"' }