[AIRFLOW-5900] avoid unnecessary system calls in heartbeat_callback

[houqp]

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Description

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Optimize hearbeat callback, avoid system calls when they are not needed.

Tests

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[codecov-io]

Codecov Report

Merging #6551 into master will decrease coverage by 0.28%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #6551      +/-   ##
==========================================
- Coverage   84.07%   83.78%   -0.29%     
==========================================
  Files         639      639              
  Lines       36900    36900              
==========================================
- Hits        31024    30917     -107     
- Misses       5876     5983     +107

Impacted Files	Coverage Δ
airflow/jobs/local_task_job.py	90% <100%> (+5%)	⬆️
airflow/kubernetes/volume_mount.py	44.44% <0%> (-55.56%)	⬇️
airflow/kubernetes/volume.py	52.94% <0%> (-47.06%)	⬇️
airflow/kubernetes/pod_launcher.py	45.25% <0%> (-46.72%)	⬇️
airflow/kubernetes/kube_client.py	33.33% <0%> (-41.67%)	⬇️
...rflow/contrib/operators/kubernetes_pod_operator.py	70.14% <0%> (-28.36%)	⬇️

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[potiuk]

Nice and small optimisation :).