Change `upload_forcing` worker to persist previous day for missing runoff files via symlinks instead of failing

[douglatornell]

re: issue #232

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (a7cdbca) 77.61% compared to head (ba31610) 77.65%. Report is 2 commits behind head on main.

:exclamation: Current head ba31610 differs from pull request most recent head e5087ac. Consider uploading reports for the commit e5087ac to get more accurate results

Files	Patch %	Lines
nowcast/workers/upload_forcing.py	77.77%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #233 +/- ## ========================================== + Coverage 77.61% 77.65% +0.04% ========================================== Files 133 133 Lines 18678 18705 +27 Branches 1899 1900 +1 ========================================== + Hits 14496 14526 +30 + Misses 4116 4113 -3 Partials 66 66 ``` | [Flag](https://app.codecov.io/gh/SalishSeaCast/SalishSeaNowcast/pull/233/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SalishSeaCast) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/SalishSeaCast/SalishSeaNowcast/pull/233/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SalishSeaCast) | `77.65% <93.33%> (+0.04%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SalishSeaCast#carryforward-flags-in-the-pull-request-comment) to find out more.

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[douglatornell]

Finally tested successfully in production on 2-Feb-2024.