proc: set pgrp in spawned daemon processes

[pmrowla]

Sets pgrp in spawned proc so signals are not forwarded from the original parent. Signal handlers are still inherited (so caller is responsible for configuring SIG_IGN/SIG_DFL/custom handlers as desired)

Fixes #60

[codecov-commenter]

Codecov Report

Merging #64 (180949c) into main (708e0a9) will increase coverage by 0.09%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##             main      #64      +/-   ##
==========================================
+ Coverage   86.49%   86.59%   +0.09%     
==========================================
  Files          19       19              
  Lines         711      716       +5     
  Branches       98      100       +2     
==========================================
+ Hits          615      620       +5     
- Misses         76       78       +2     
+ Partials       20       18       -2     

Impacted Files	Coverage Δ
src/dvc_task/proc/process.py	84.54% <33.33%> (-2.13%)	⬇️
src/dvc_task/worker/temporary.py	87.93% <0.00%> (+5.17%)	⬆️

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