[AIRFLOW-6933] Pass in env vars for all operators

[saguziel]

Pass in the airflow context env vars for all operators by doing it in ti._run_raw. Remove it from PythonOperator since it is unnecessary. Leave it in other places since they do special insertions.

Issue link: AIRFLOW-6933

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[codecov-io]

Codecov Report

Merging #7576 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #7576      +/-   ##
==========================================
+ Coverage   86.56%   86.57%   +0.01%     
==========================================
  Files         896      896              
  Lines       42622    42626       +4     
==========================================
+ Hits        36896    36905       +9     
+ Misses       5726     5721       -5

Impacted Files	Coverage Δ
airflow/operators/python.py	95% <ø> (-0.14%)	⬇️
airflow/models/taskinstance.py	95.33% <100%> (+0.15%)	⬆️
airflow/jobs/scheduler_job.py	90.46% <0%> (-0.03%)	⬇️
airflow/cli/commands/version_command.py	100% <0%> (ø)	⬆️
airflow/utils/dag_processing.py	86.65% <0%> (+0.05%)	⬆️
...rflow/providers/apache/spark/hooks/spark_submit.py	84.67% <0%> (+0.05%)	⬆️
airflow/models/dag.py	91.57% <0%> (+0.06%)	⬆️
airflow/jobs/backfill_job.py	92.15% <0%> (+1.12%)	⬆️

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