fix: included variable merging (#1649)

testdata/included_taskfile_var_merging/Taskfile.yaml

@@ -0,0 +1,12 @@
+version: "3"
+
+includes:
+  foo:
+    taskfile: ./foo/Taskfile.yaml
+  bar:
+    taskfile: ./bar/Taskfile.yaml
+
+tasks:
+  stub:
+    cmds:
+      - echo 0

testdata/included_taskfile_var_merging/bar/Taskfile.yaml

@@ -0,0 +1,11 @@
+version: "3"
+
+vars:
+  DIR: bar
+
+tasks:
+  pwd:
+    dir: ./{{ .DIR }}
+    cmds:
+      - echo "{{ .DIR }}"
+      - pwd

testdata/included_taskfile_var_merging/foo/Taskfile.yaml

@@ -0,0 +1,11 @@
+version: "3"
+
+vars:
+  DIR: foo
+
+tasks:
+  pwd:
+    dir: ./{{ .DIR }}
+    cmds:
+      - echo "{{ .DIR }}"
+      - pwd