[ci] feat: Set Megatron related environment variable with ENV in Ascend dockerfile

[FightingZhen]

What does this PR do?

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As title, the purpose of this PR is to replace the current practice of writing environment variables into bashrc with using ENV instructions to store Megatron-related environment variables in the Ascend Dockerfile.

This optimization ensures that environment variables take effect in both shell-based usage scenarios (e.g., shell interfaces) and non-shell-based scenarios (e.g., e2e_ascend), eliminating the need to set additional environment variable instructions in the e2e_ascend configuration file.

Note: Removing export commands in e2e_ascend will be committed in next PR, this PR only aims at updating dockerfile and corresponding images after PR merged.

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[gemini-code-assist]

Code Review

This pull request correctly moves the setting of the PYTHONPATH environment variable from .bashrc to an ENV instruction in the Dockerfiles. This ensures the variable is set for both interactive and non-interactive sessions. However, the current implementation of the ENV instruction could introduce a security vulnerability by potentially adding the current directory to Python's module search path. I've added suggestions to all four modified Dockerfiles to address this by conditionally prepending to PYTHONPATH only if it's already set.