[IRC] Set up a backup nickname for the github-event-bot

[rl2939]

Description

Running two or more pull requests near each other causes the github-event-bot to malfunction and causes the IRC part of the build in the pull request to fail.

< :molybdenum.libera.chat 433 * github-event-bot :Nickname is already in use.

From https://github.com/DISTRHO/Cardinal/actions/runs/11851548872/job/33028220811?pr=739:

Run Gottox/irc-message-action@v2
# NetTransport socketRawConnected()
# NetTransport socketFullyConnected()
# Socket fully connected
> CAP LS 302
> NICK github-event-bot
> USER ircbot 0 * ircbot
< :molybdenum.libera.chat NOTICE * :*** Checking Ident
< :molybdenum.libera.chat NOTICE * :*** Looking up your hostname...
< :molybdenum.libera.chat NOTICE * :*** Couldn't look up your hostname
< :molybdenum.libera.chat NOTICE * :*** No Ident response
< :molybdenum.libera.chat CAP * LS :account-notify away-notify chghost extended-join multi-prefix sasl=ECDSA-NIST256P-CHALLENGE,EXTERNAL,PLAIN,SCRAM-SHA-512 tls account-tag cap-notify echo-message server-time solanum.chat/identify-msg solanum.chat/oper solanum.chat/realhost
> CAP REQ :account-notify away-notify extended-join multi-prefix account-tag cap-notify server-time
< :molybdenum.libera.chat 433 * github-event-bot :Nickname is already in use.
< :molybdenum.libera.chat CAP * ACK :account-notify away-notify extended-join multi-prefix account-tag cap-notify server-time
> CAP END
< ERROR :Closing Link: 172.183.162.199 (Connection timed out)
# NetTransport socketClose()
# Socket closed. was_connected=true safely_registered=false requested_disconnect=false