fix: support a SOURCE_DATE_EPOCH prior to 1980

[branchvincent]

A common value for SOURCE_DATE_EPOCH is 0, which unfortunately breaks pdm:

$ uv init --build-backend=pdm
$ SOURCE_DATE_EPOCH=0 uv build
# ...
  File "/Users/branch/.cache/uv/builds-v0/.tmpEkVowp/lib/python3.13/site-packages/pdm/backend/wheel.py", line 52, in <module>
    raise ValueError("zipinfo date can't be earlier than 1980")
ValueError: zipinfo date can't be earlier than 1980

This clamps to 1980 instead (same as setuptools and flit-core. poetry also support this but defaults to 2016)

[netlify]

✅ Deploy Preview for pdm-backend-docs ready!

Name	Link
🔨 Latest commit	9d1c18d
🔍 Latest deploy log	https://app.netlify.com/projects/pdm-backend-docs/deploys/68491b9d830b84000964e3fe
😎 Deploy Preview	https://deploy-preview-288--pdm-backend-docs.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[branchvincent]

thanks for the review @frostming! any idea on when this might be merged + released?