Add ability to use -betapassword option for SteamCMD branches

[cydactyl]

User Story

As an Arma 3 server admin, I want to be able to use the profiling branch easily with Linux GSM so that I can have better performance and keep up-to-date.

Basic info

• Distro: [Ubuntu 18.04]
• Game: [Arma 3]
• Command: [update]
• LinuxGSM version: [v20.1.5]

Further Information

Add -betapassword option to SteamCMD update process and various SteamCMD based server config files

Further Reading

https://developer.valvesoftware.com/wiki/SteamCMD#Downloading_an_app

https://community.bistudio.com/wiki/Arma_3_Steam_Branches#Arma_3_Server

https://github.com/GameServerManagers/LinuxGSM/blob/v20.1.5/lgsm/functions/update_steamcmd.sh

[cydactyl]

I'm not too familiar with the codebase but the only issue I see is not being able to specify an empty -betapassword option without failure. A simple if statement to omit -betapassword if (betapassword="") would probably suffice.

[github-actions]

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