libretiny: treat already-patched .ld as success on incremental rebuilds

The idempotency check in _inject_keep returns content unchanged when the
marker is already present, so patched stayed 0 and the fail-hard
RuntimeError fired on every incremental rebuild after the first. Check
for the marker before attempting to patch and count it as success.

esphome/components/libretiny/patch_linker.py.script

@@ -86,6 +86,9 @@ def _pre_link(target, source, env):
         path = os.path.join(build_dir, name)
         with open(path, "r", encoding="utf-8") as fh:
             original = fh.read()
+        if _MARKER in original:
+            patched += 1
+            continue
         content = original
         for fn in _patchers:
             content = fn(content)