Update to Rust 1.86.0

[Stebalien]

This fixes a crash we were seeing in CI in v1.81.0.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 76.77%. Comparing base (7a45269) to head (88d99e9).

Files with missing lines	Patch %	Lines
shared/src/sys/mod.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2121      +/-   ##
==========================================
- Coverage   76.83%   76.77%   -0.07%     
==========================================
  Files         149      149              
  Lines       15643    15631      -12     
==========================================
- Hits        12019    12000      -19     
- Misses       3624     3631       +7     

Files with missing lines	Coverage Δ
fvm/src/engine/mod.rs	86.14% <100.00%> (ø)
fvm/src/machine/default.rs	69.47% <ø> (ø)
fvm/src/syscalls/event.rs	100.00% <ø> (ø)
fvm/src/syscalls/linker.rs	93.87% <ø> (ø)
fvm/src/syscalls/send.rs	100.00% <100.00%> (ø)
ipld/amt/src/diff.rs	85.52% <ø> (ø)
ipld/car/src/util.rs	80.39% <100.00%> (-0.38%)	⬇️
ipld/encoding/src/bytes.rs	79.66% <ø> (ø)
ipld/hamt/src/iter.rs	92.30% <ø> (ø)
ipld/hamt/src/node.rs	91.03% <100.00%> (+0.17%)	⬆️
... and 5 more

... and 10 files with indirect coverage changes

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[Stebalien]

We're going to need to be careful about updating rust in the builtin actors, but we'll need to do it eventually so we might as well do this now.

[rvagg]

need to be careful about updating rust in the builtin actors

Can you describe how you would go about being careful other than pushing through and using existing integration tests to see what breaks? I guess digging through changelogs would be a start, but what else would you look for/at?

[Stebalien]

filecoin-project/builtin-actors#1650 (comment)