Fix chain-depth-2 zcap invocation expiry check.

[dlongley]

No description provided.

[codecov-commenter]

Codecov Report

Merging #82 (8067dca) into main (d93b28c) will increase coverage by 0.08%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #82      +/-   ##
==========================================
+ Coverage   90.73%   90.82%   +0.08%     
==========================================
  Files           6        6              
  Lines        1868     1886      +18     
==========================================
+ Hits         1695     1713      +18     
  Misses        173      173              

Files	Coverage Δ
lib/CapabilityInvocation.js	85.67% <100.00%> (+0.84%)	⬆️
lib/CapabilityProofPurpose.js	98.49% <100.00%> (ø)
lib/utils.js	86.21% <100.00%> (-0.03%)	⬇️

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