t5562: avoid non-portable "export FOO=bar" construct

Commit 6c213e863a ("http-backend: respect CONTENT_LENGTH for
receive-pack", 2018-07-27) adds a test which uses the non-portable
export construct. Replace it with "FOO=bar && export FOO" instead.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5562-http-backend-content-length.sh

@@ -45,7 +45,8 @@ ssize_b100dots() {
 }
 
 test_expect_success 'setup' '
-	export HTTP_CONTENT_ENCODING="identity" &&
+	HTTP_CONTENT_ENCODING="identity" &&
+	export HTTP_CONTENT_ENCODING &&
 	git config http.receivepack true &&
 	test_commit c0 &&
 	test_commit c1 &&