Use rustc stable for benchmark CI run to fix current breakage

[TheBlueMatt]

No description provided.

[codecov-commenter]

Codecov Report

Base: 90.72% // Head: 90.71% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (2ef3d18) compared to base (6957fb6).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1818      +/-   ##
==========================================
- Coverage   90.72%   90.71%   -0.02%     
==========================================
  Files          87       87              
  Lines       47361    47361              
  Branches    47361    47361              
==========================================
- Hits        42968    42962       -6     
- Misses       4393     4399       +6     

Impacted Files	Coverage Δ
lightning/src/chain/onchaintx.rs	94.61% <0.00%> (-0.68%)	⬇️
lightning/src/ln/functional_tests.rs	96.94% <0.00%> (-0.06%)	⬇️
lightning/src/ln/peer_channel_encryptor.rs	93.62% <0.00%> (+0.24%)	⬆️

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