fix target_index in packet log writer

[ryanmelt]

target_index was incorrectly incrementing with each new log file created by the same PacketLogWriter rendering it invalid.

[codecov]

Codecov Report

Patch coverage: 40.00% and project coverage change: -0.08 ⚠️

Comparison is base (a78b55f) 74.35% compared to head (5e75ecc) 74.28%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #617      +/-   ##
==========================================
- Coverage   74.35%   74.28%   -0.08%     
==========================================
  Files         460      460              
  Lines       28541    28546       +5     
  Branches      588      588              
==========================================
- Hits        21222    21205      -17     
- Misses       7226     7247      +21     
- Partials       93       94       +1     

Flag	Coverage Δ
frontend	74.70% <ø> (-0.40%)	⬇️
ruby-api	50.96% <ø> (ø)
ruby-backend	78.44% <40.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
openc3/lib/openc3/logs/packet_log_reader.rb	89.02% <25.00%> (-1.61%)	⬇️
openc3/lib/openc3/logs/packet_log_writer.rb	99.01% <100.00%> (+<0.01%)	⬆️

... and 5 files with indirect coverage changes

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[jmthomas]

Great new test case!