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]

Codecov Report

Merging #28 (ca063a9) into master (8a04318) will decrease coverage by 0.69%.
The diff coverage is 66.66%.

@@             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     

Impacted Files	Coverage Δ
backup.sh	99.30% <66.66%> (-0.70%)	⬇️

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