fix(external): use URLEncoding to decode blob URL (#1047)

Because

- base64.StdEncoding might generate `/` in the Base64 string, which can
break the structure of the generated blob URL.

This commit:

- Uses base64.URLEncoding to decode the blob URL.

Author: donch1989

pkg/external/external.go

@@ -300,7 +300,7 @@ func (f *artifactBinaryFetcher) FetchFromURL(ctx context.Context, fileURL string
 		// The presigned URL is encoded in the format:
 		// scheme://host/v1alpha/blob-urls/base64_encoded_presigned_url
 		// Here we decode the base64 string to the presigned URL.
-		base64Decoded, err := base64.StdEncoding.DecodeString(strings.Split(parsedURL.Path, "/")[3])
+		base64Decoded, err := base64.URLEncoding.DecodeString(strings.Split(parsedURL.Path, "/")[3])
 		if err != nil {
 			return nil, "", "", err
 		}