add socket option for tmux window manager

[tstoco]

I had to use different sockets on my systemd deployment and the script was not sending messages to players on the server.

[nicolaschan]

Thanks for your contribution! It looks like it is failing some of the above checks. I'll merge this PR once it's been updated to pass those checks.

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 66.66667% with 2 lines in your changes missing coverage. Please review.

Project coverage is 99.30%. Comparing base (8a04318) to head (ca063a9).
Report is 13 commits behind head on master.

Files with missing lines	Patch %	Lines
backup.sh	66.66%	2 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##            master      #28      +/-   ##
===========================================
- Coverage   100.00%   99.30%   -0.70%     
===========================================
  Files            1        1              
  Lines          284      289       +5     
===========================================
+ Hits           284      287       +3     
- Misses           0        2       +2     

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