--charset is, unfortunately, a bit of a misnomer -- it should really be called --connection-charset. I think the solution here is simply changing the --alter statement to
--alter "CHARACTER SET utf8, MODIFY email VARCHAR(128) CHARACTER SET utf8 COLLATE utf8_unicode_ci"
Which will modify the charset of the temp table to utf8.
--charset is, unfortunately, a bit of a misnomer -- it should really be called --connection- charset. I think the solution here is simply changing the --alter statement to
--alter "CHARACTER SET utf8, MODIFY email VARCHAR(128) CHARACTER SET utf8 COLLATE utf8_unicode_ci"
Which will modify the charset of the temp table to utf8.