update publish workflow

[themattinthehatt]

gsplat is a git dependency that requires torch to build its C extensions. In the publish workflow, the poetry install --only-root step triggers dependency resolution/building for gsplat, but torch isn't available yet.

This PR removes the poetry install --only-root step entirely. It's not needed — poetry build just packages the source files into a wheel/sdist without needing to install or compile any dependencies. gsplat will only be compiled when users install beast-backbones on their own machines (where torch is already present).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.51%. Comparing base (6058ec9) to head (2e3cbbc).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #58   +/-   ##
=======================================
  Coverage   86.51%   86.51%           
=======================================
  Files          55       55           
  Lines        5022     5022           
=======================================
  Hits         4345     4345           
  Misses        677      677           

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