fix Flow deserialization for hosts

[gpetretto]

Summary

There was an issue with the (de)serialization of a Flow. Since the hosts are added when a Flow is created from a list of Flows/Jobs, during deserialization hosts were duplicated for each of the jobs contained in the Flow.

This PR should fix this.

[codecov[bot]]

Codecov Report

Merging #499 (46e10db) into main (08fbf5f) will increase coverage by 0.00%. Report is 9 commits behind head on main. The diff coverage is 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #499 +/- ## ======================================= Coverage 99.86% 99.86% ======================================= Files 20 20 Lines 1509 1510 +1 Branches 415 416 +1 ======================================= + Hits 1507 1508 +1 Misses 2 2 ``` | [Files](https://app.codecov.io/gh/materialsproject/jobflow/pull/499?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=materialsproject) | Coverage Δ | | |---|---|---| | [src/jobflow/core/flow.py](https://app.codecov.io/gh/materialsproject/jobflow/pull/499?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=materialsproject#diff-c3JjL2pvYmZsb3cvY29yZS9mbG93LnB5) | `100.00% <100.00%> (ø)` | |

[utf]

Great, thank you.