gh-96954: Fix `make regen-unicodedata` in out-of-tree builds (#112118)

This avoids:

    python3.13 Tools/unicode/makeunicodedata.py
    python3.13: can't open file '.../build/debug/Tools/unicode/makeunicodedata.py': [Errno 2] No such file or directory
    make: *** [Makefile:1498: regen-unicodedata] Error 2

Re-run `make regen-unicodedata` to update the script path in generated files.