[substitutions] Fix #7189 (#9469)

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

@@ -17,3 +17,5 @@ test_list:
   - ${undefined_var}
   - $undefined_var
   - ${ undefined_var }
+  - key1: 1
+    key2: 2

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

@@ -19,3 +19,5 @@ test_list:
   - ${undefined_var}
   - $undefined_var
   - ${ undefined_var }
+  - key${var1}: 1
+    key${var2}: 2

tests/unit_tests/fixtures/substitutions/03-closures.approved.yaml

@@ -6,6 +6,7 @@ package_result:
     root file
   - Double substitution also works; the value of var7 is 79, where A is a package
     var
+  - key79: Key should substitute to key79
 local_results:
   - The value of B is 5
   - 'You will see, however, that

tests/unit_tests/fixtures/substitutions/closures_package.yaml

@@ -1,3 +1,4 @@
 package_result:
   - The value of A*B is ${A * B}, where A is a package var and B is a substitution in the root file
   - Double substitution also works; the value of var7 is ${var$A}, where A is a package var
+  - key${var7}: Key should substitute to key79