improve --env handling in CLI job run

[shcheklein]

Now we can pass:

--env something=value \
--env something_else=value1

to datachain job run - it confused me, it confused AI, etc (it was silently just using the last entry)

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[Copilot]

Pull request overview

This PR improves the --env flag handling for the datachain job run command to properly support multiple environment variable declarations. Previously, when multiple --env flags were provided, only the last one would be used (silently ignoring others). The fix allows users to specify environment variables using either multiple --env flags or multiple values in a single --env flag.

Changes:

• Added action="append" to the --env argument parser to accumulate all --env flag values
• Implemented flattening logic to properly handle the resulting list of lists before joining into environment string
• Updated tests to verify both usage patterns (multiple flags and multiple values per flag)
• Enhanced documentation with examples showing both supported patterns

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/datachain/cli/parser/job.py	Added action="append" to the --env argument to accumulate multiple flag instances
src/datachain/studio.py	Added flatten import and logic to properly process the nested list structure from argparse
tests/test_cli_studio.py	Updated test to validate both single --env with multiple values and multiple --env flags
docs/commands/job/run.md	Added new example demonstrating both patterns for specifying multiple environment variables and renumbered subsequent examples

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Codecov Report

✅ All modified and coverable lines are covered by tests.

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[amritghimire]

Tests seems to be failing, can you please take a look?

[shcheklein]

@amritghimire unrelated, tocrchcodec, but yes, need to be addressed before merging ... I thought @ilongin fixed it tbh today, but it seems it something different.

[dreadatour]

👍