otk: handle missing suport for joins with vars

This commit fixes the issue that the following snippet:
```
otk.version: 1

otk.define:
  default:
    modifications:
      language: en_US.UTF-8

  user:
    modifications:
      language: nl_NL.UTF-8

  modifications:
    otk.op.join:
      values:
        - ${default.modifications}
        - ${user.modifications}

otk.target.osbuild:
  lang: ${modifications.language}
```
does not work because no var substitution was performed in the
join.

test/data/base/19-op-join-with-vars.json

@@ -0,0 +1,4 @@
+{
+  "lang":"nl_NL.UTF-8",
+  "version": "2"
+}

test/data/base/19-op-join-with-vars.yaml

@@ -0,0 +1,19 @@
+otk.version: 1
+
+otk.define:
+  default:
+    modifications:
+      language: en_US.UTF-8   
+
+  user:
+    modifications:
+      language: nl_NL.UTF-8
+
+  modifications:
+    otk.op.join:
+      values:
+        - ${default.modifications}
+        - ${user.modifications}
+
+otk.target.osbuild:
+  lang: ${modifications.language}