fixes #20572 (#20585)

* fixes #20572

* added a test case

tests/arc/texplicit_sink.nim

@@ -0,0 +1,29 @@
+discard """
+  output: '''de'''
+  cmd: '''nim c --mm:arc --expandArc:main $file'''
+  nimout: '''--expandArc: main
+
+var
+  a
+  b_cursor
+try:
+  a = f "abc"
+  b_cursor = "de"
+  `=sink`(a, b_cursor)
+  echo [a]
+finally:
+  `=destroy`(a)
+-- end of expandArc ------------------------'''
+"""
+
+# bug #20572
+
+proc f(s: string): string = s
+
+proc main =
+  var a = f "abc"
+  var b = "de"
+  `=sink`(a, b)
+  echo a
+
+main()