Always export variables

After some testing, I decided that we should not reset the variables.
Because if we do that, the information becomes unavailable to the
preview process as it runs asynchronously in a different goroutine.

  fzf --header-lines=2 --bind 'click-header:transform-prompt(printf ${FZF_CLICK_HEADER_LINE}x${FZF_CLICK_HEADER_COLUMN})+preview(env | grep FZF)'

src/terminal.go

@@ -847,10 +847,6 @@ func (t *Terminal) environ() []string {
 	if t.listenPort != nil {
 		env = append(env, fmt.Sprintf("FZF_PORT=%d", *t.listenPort))
 	}
-	if t.clickHeaderLine > 0 {
-		env = append(env, fmt.Sprintf("FZF_CLICK_HEADER_LINE=%d", t.clickHeaderLine))
-		env = append(env, fmt.Sprintf("FZF_CLICK_HEADER_COLUMN=%d", t.clickHeaderColumn))
-	}
 	env = append(env, "FZF_QUERY="+string(t.input))
 	env = append(env, "FZF_ACTION="+t.lastAction.Name())
 	env = append(env, "FZF_KEY="+t.lastKey)
@@ -863,6 +859,8 @@ func (t *Terminal) environ() []string {
 	env = append(env, fmt.Sprintf("FZF_LINES=%d", t.areaLines))
 	env = append(env, fmt.Sprintf("FZF_COLUMNS=%d", t.areaColumns))
 	env = append(env, fmt.Sprintf("FZF_POS=%d", util.Min(t.merger.Length(), t.cy+1)))
+	env = append(env, fmt.Sprintf("FZF_CLICK_HEADER_LINE=%d", t.clickHeaderLine))
+	env = append(env, fmt.Sprintf("FZF_CLICK_HEADER_COLUMN=%d", t.clickHeaderColumn))
 	return env
 }
 
@@ -4041,11 +4039,7 @@ func (t *Terminal) Loop() {
 						if my >= 0 && my < numLines && mx >= 0 {
 							t.clickHeaderLine = my + 1
 							t.clickHeaderColumn = mx + 1
-							evt := tui.ClickHeader
-							res := doActions(actionsFor(evt))
-							t.clickHeaderLine = 0
-							t.clickHeaderColumn = 0
-							return res
+							return doActions(actionsFor(tui.ClickHeader))
 						}
 					}
 				}