skip sending individual files to export remotes

That will fail, and it already exports whole trees.
f6dd34ca81 made it sync content with
import remotes, and if an import remote is also an export remote, that
caused this new failure mode.

Sponsored-by: Brock Spratlen on Patreon

Command/Sync.hs

@@ -296,9 +296,9 @@ seek' o = do
 						(filter isExport contentremotes)
 
 				-- Sync content with remotes, including
-				-- with import remotes (since importing
-				-- only downloads new files not old files),
-				-- but not with export only remotes.
+				-- importing from import remotes (since
+				-- importing only downloads new files not
+				-- old files)
 				let shouldsynccontent r
 					| isExport r && not (isImport r) = False
 					| otherwise = True
@@ -926,6 +926,7 @@ syncFile o ebloom rs af k = do
 	wantput r
 		| pushOption o == False = return False
 		| Remote.readonly r || remoteAnnexReadOnly (Remote.gitconfig r) = return False
+		| isExport r = return False
 		| isThirdPartyPopulated r = return False
 		| otherwise = wantGetBy True (Just k) af (Remote.uuid r)
 	handleput lack inhere