tests: remove invalid escape sequences

>>> "\@"
<stdin>:1: SyntaxWarning: invalid escape sequence '\@'
'\\@'

tests/clamav.nix

@@ -222,7 +222,7 @@ pkgs.nixosTest {
 
       with subtest("virus scan email"):
           client.succeed(
-              'set +o pipefail; msmtp -a user2 user1\@example.com < /etc/root/virus-email 2>&1 | tee /dev/stderr | grep "server message: 554 5\\.7\\.1" >&2'
+              'set +o pipefail; msmtp -a user2 user1@example.com < /etc/root/virus-email 2>&1 | tee /dev/stderr | grep "server message: 554 5\\.7\\.1" >&2'
           )
           server.succeed("journalctl -u rspamd | grep -i eicar")
           # give the mail server some time to process the mail