[MRG] fix sig check manifest rebuilding

[ctb]

Per #1907 (comment), fix sig check to not rebuild manifests every time! 🙄

[ctb]

(should be ready for review & merge @bluegenes)

[codecov]

Codecov Report

Merging #1915 (afe243d) into latest (3b267fe) will increase coverage by 7.87%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1915      +/-   ##
==========================================
+ Coverage   82.96%   90.83%   +7.87%     
==========================================
  Files         123       93      -30     
  Lines       13365     9165    -4200     
  Branches     1808     1808              
==========================================
- Hits        11088     8325    -2763     
+ Misses       2013      576    -1437     
  Partials      264      264              

Flag	Coverage Δ
python	90.83% <100.00%> (+<0.01%)	⬆️
rust	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/sourmash/cli/sig/check.py	100.00% <100.00%> (ø)
src/sourmash/sig/__main__.py	93.56% <100.00%> (ø)
src/sourmash/sourmash_args.py	93.40% <100.00%> (+0.01%)	⬆️
src/core/src/index/linear.rs
src/core/src/storage.rs
src/core/src/errors.rs
src/core/src/from.rs
src/core/src/index/sbt/mhbt.rs
src/core/src/index/revindex.rs
src/core/src/ffi/cmd/compute.rs
... and 23 more

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[bluegenes]

👍🏻!