[AIRFLOW-5772] Use mock.patch.dict for manipulation of env vars

[mik-laj]

Side effects when modifying the os.environ variable are often a problem. Using unittest.mock ensures test security and stability, because it guarantees that it restore the dictionary to its original state after the test.

More information: https://docs.python.org/3/library/unittest.mock.html#unittest.mock.patch.dict

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[codecov-io]

Codecov Report

Merging #6444 into master will decrease coverage by 0.45%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6444      +/-   ##
==========================================
- Coverage   84.15%   83.69%   -0.46%     
==========================================
  Files         627      627              
  Lines       36537    36537              
==========================================
- Hits        30746    30579     -167     
- Misses       5791     5958     +167

Impacted Files	Coverage Δ
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%)	⬇️
airflow/gcp/example_dags/example_vision.py
airflow/gcp/operators/vision.py
airflow/gcp/hooks/vision.py
airflow/providers/google/cloud/hooks/vision.py	92.09% <0%> (ø)
...viders/google/cloud/example_dags/example_vision.py	0% <0%> (ø)
... and 6 more

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[turbaszek]

LGTM @mik-laj 👌🏼

[kaxil]

LGTM