feat: allow instances to add extra translation sources (#37962)

[deborahgu]

allows instances to set the variable ATLAS_EXTRA_SOURCES so they can add their own sources to make pull_translations.

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Pull request overview

This PR makes the make pull_translations target extensible so instances can add their own Atlas translation sources via an environment variable.

Changes:

• Extends the atlas pull command in the pull_translations Makefile target to append sources defined in ATLAS_EXTRA_SOURCES.
• Keeps existing translation sources intact while allowing instance-level customization without modifying core Makefile logic.

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