The generated-files builder timeout cannot be changed #1306

Where: scripts/check_generated_files_current.py (BUILD_TIMEOUT).

Current gap: The 600 s limit for each regeneration step is a constant, so a slow CI runner or a very large explainer set needs a code change (the gh timeout got FAIRCODE_GH_TIMEOUT for this reason).

Suggested approach: Read an optional FAIRCODE_BUILD_TIMEOUT (seconds), ignore values that are not positive numbers and mention it in the module docstring.

Done when: a test sets the variable and sees the limit used, and an invalid value falls back to 600.