Adding feature in bash operator to append the user defined env variable to system env variable

[PraveenA95]

Feature to override user defined env variables on System Env Variables

Currently in bash operator whenever a user passes a environment variables using env argument, the user defined environment variables overrides the existing system environment variables. In this PR, I have introduced a new argument called append_env. The default value for this will be False. When append_env is set True then we will update the system environment variables with user defined environment variables(new Behavior), If set False, we ignore the system environment variables and use the user defined variables(Existing Behavior)

Closes : #17240

[boring-cyborg]

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[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest main at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.

[boring-cyborg]

Awesome work, congrats on your first merged pull request!