[MRG] set pickfile attribute on SourmashPicklist.load(...)

[ctb]

The load method on SourmashPicklist did not set the pickfile attribute, while the class method from_picklist_args(...) did set it. This PR fixes load to be consistent with from_picklist_args.

Fixes #1773

[codecov]

Codecov Report

Merging #1776 (ffcef91) into latest (c9c0303) will increase coverage by 6.67%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1776      +/-   ##
==========================================
+ Coverage   83.42%   90.10%   +6.67%     
==========================================
  Files         113       87      -26     
  Lines       12140     8440    -3700     
  Branches     1614     1614              
==========================================
- Hits        10128     7605    -2523     
+ Misses       1753      576    -1177     
  Partials      259      259              

Flag	Coverage Δ
python	90.10% <100.00%> (+<0.01%)	⬆️
rust	?

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Impacted Files	Coverage Δ
src/sourmash/picklist.py	90.51% <100.00%> (+0.06%)	⬆️
src/core/src/index/sbt/mhbt.rs
src/core/src/index/mod.rs
src/core/src/errors.rs
src/core/src/sketch/nodegraph.rs
src/core/src/ffi/minhash.rs
src/core/src/signature.rs
src/core/src/ffi/utils.rs
src/core/src/index/search.rs
src/core/tests/test.rs
... and 17 more

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[ctb]

ready for review @sourmash-bio/devs - the failing dev envs test is not caused by this change, and is being investigated #1778