Skip imgconverter availability check if builder supports the image type

Close sphinx-doc#7973: ImgConverter runs is_available in HTML builder
Close missinglinkelectronics/sphinxcontrib-svg2pdfconverter#8: Extension should only run on LaTeX builder

sphinx/transforms/post_transforms/images.py

@@ -197,15 +197,15 @@ def __init__(self, *args: Any, **kwargs: Any) -> None:
     def match(self, node: nodes.image) -> bool:
         if not self.app.builder.supported_image_types:
             return False
+        elif set(node['candidates']) & set(self.app.builder.supported_image_types):
+            # builder supports the image; no need to convert
+            return False
         elif self.available is None:
             # store the value to the class variable to share it during the build
             self.__class__.available = self.is_available()
 
         if not self.available:
             return False
-        elif set(node['candidates']) & set(self.app.builder.supported_image_types):
-            # builder supports the image; no need to convert
-            return False
         else:
             rule = self.get_conversion_rule(node)
             if rule: