Make dynamic variables run on the right directory

It was always running in the main Taskfile dir, even when the variable was
declared in an included taskfile in another directory or when task had a
custom dir.

Closes #384

testdata/dir/dynamic_var/.gitignore

@@ -0,0 +1 @@
+subdirectory/dir.txt

testdata/dir/dynamic_var/Taskfile.yml

@@ -0,0 +1,11 @@
+version: '3'
+
+tasks:
+  default:
+    cmds:
+      - echo '{{.FOLDER}}' > dir.txt
+    dir: subdirectory
+    vars:
+      FOLDER:
+        sh: basename $(pwd)
+    silent: true

testdata/dir/dynamic_var/subdirectory/dir.txt

@@ -0,0 +1 @@
+subdirectory

testdata/generates/Taskfile.yml

@@ -4,7 +4,7 @@ vars:
   BUILD_DIR: $pwd
 
 tasks:
-  abs.txt:
+  sub/abs.txt:
     desc: generates dest file based on absolute paths
     deps:
       - sub/src.txt