Fix templated env_vars field in KubernetesPodOperator to allow for compatibility with XComArgs

[nyoungstudios]

Allows for usage of XComArgs for env_vars field in KubernetesPodOperator by moving the raise error conditional statement to after the jinja template strings are rendered.
Also by extension, allows for usage of passing a jinja template string to the env_vars field when render_as_native_obj=True converts it to a dictionary

related discussion: #38522
The example dag on the related discussion now works

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[boring-cyborg]

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[hussein-awala]

The change looks good, let's wait for unit tests.
LGTM

[boring-cyborg]

Awesome work, congrats on your first merged pull request! You are invited to check our Issue Tracker for additional contributions.

[potiuk]

NICE.