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

task_test.go

@@ -303,16 +303,15 @@ func TestPrecondition(t *testing.T) {
 }
 
 func TestGenerates(t *testing.T) {
+	const dir = "testdata/generates"
+
 	const (
 		srcTask        = "sub/src.txt"
 		relTask        = "rel.txt"
-		absTask        = "abs.txt"
+		absTask        = "sub/abs.txt"
 		fileWithSpaces = "my text file.txt"
 	)
 
-	// This test does not work with a relative dir.
-	dir, err := filepath.Abs("testdata/generates")
-	assert.NoError(t, err)
 	var srcFile = filepath.Join(dir, srcTask)
 
 	for _, task := range []string{srcTask, relTask, absTask, fileWithSpaces} {
@@ -800,6 +799,18 @@ func TestWhenDirAttributeItCreatesMissingAndRunsInThatDir(t *testing.T) {
 	_ = os.RemoveAll(toBeCreated)
 }
 
+func TestDynamicVariablesShouldRunOnTheTaskDir(t *testing.T) {
+	tt := fileContentTest{
+		Dir:       "testdata/dir/dynamic_var",
+		Target:    "default",
+		TrimSpace: false,
+		Files: map[string]string{
+			"subdirectory/dir.txt": "subdirectory\n",
+		},
+	}
+	tt.Run(t)
+}
+
 func TestDisplaysErrorOnUnsupportedVersion(t *testing.T) {
 	e := task.Executor{
 		Dir:    "testdata/version/v1",