Don't run `addurl` until it's needed

[jwodder]

Part of #347.

[codecov[bot]]

Codecov Report

Patch coverage: 56.60% and project coverage change: +0.01 :tada:

Comparison is base (8642675) 76.23% compared to head (8cd2700) 76.24%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## draft #351 +/- ## ========================================== + Coverage 76.23% 76.24% +0.01% ========================================== Files 14 14 Lines 2470 2488 +18 Branches 544 550 +6 ========================================== + Hits 1883 1897 +14 - Misses 435 438 +3 - Partials 152 153 +1 ``` | [Impacted Files](https://app.codecov.io/gh/dandi/dandisets/pull/351?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi) | Coverage Δ | | |---|---|---| | [tools/backups2datalad/adataset.py](https://app.codecov.io/gh/dandi/dandisets/pull/351?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi#diff-dG9vbHMvYmFja3VwczJkYXRhbGFkL2FkYXRhc2V0LnB5) | `79.72% <27.27%> (ø)` | | | [tools/backups2datalad/asyncer.py](https://app.codecov.io/gh/dandi/dandisets/pull/351?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dandi#diff-dG9vbHMvYmFja3VwczJkYXRhbGFkL2FzeW5jZXIucHk=) | `79.89% <64.28%> (-0.11%)` | :arrow_down: |

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[yarikoptic]

will pull/cleanup for the next cron to try