Delay static manifest until our head is within the target period

[Stebalien]

Unfortunately, due to filecoin-project/lotus#12557, time in our lotus tests is basically broken. So we start by relying on time but also check the head so we don't switch too early.

This doesn't matter in practice, but I'd really like to be able to test this in CI, fully integrated.

[codecov]

Codecov Report

Attention: Patch coverage is 21.42857% with 11 lines in your changes missing coverage. Please review.

Project coverage is 75.91%. Comparing base (2731818) to head (0a3aa14).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
manifest/fusing_provider.go	21.42%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #696      +/-   ##
==========================================
- Coverage   76.58%   75.91%   -0.68%     
==========================================
  Files          69       69              
  Lines        5582     5596      +14     
==========================================
- Hits         4275     4248      -27     
- Misses        896      930      +34     
- Partials      411      418       +7     

Files with missing lines	Coverage Δ
manifest/fusing_provider.go	68.49% <21.42%> (-11.17%)	⬇️

... and 7 files with indirect coverage changes