Ignore doc-snapshot outputs on non-nightly compiler

[TimDiekmann]

🌟 What is the purpose of this PR?

The line-number in doc-test changed in a recent nightly compiler version so the lines do not align on 1.65 and the current nightly compiler. This only runs the snapshot tests on a nightly toolchain. There are still plenty of tests, which are covered on the stable toolchain.

🔗 Related links

• Update Rust toolchains for /libs to nightly-2023-05-08 #2521

🔍 What does this change?

• Only run doc-snapshot tests on nightly

Pre-Merge Checklist 🚀

🚢 Has this modified a publishable library?

This PR:

• modifies a Cargo-publishable library, but it is not yet ready to publish

📜 Does this require a change to the docs?

The changes in this PR:

• does not require a docs change

🕸️ Does this require a change to the Turbo Graph?

The changes in this PR:

• does not affect the execution graph

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 55.66%. Comparing base (c794800) to head (1caf10d).
Report is 2953 commits behind head on main.

Files with missing lines	Patch %	Lines
libs/error-stack/src/fmt/hook.rs	0.00%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2528   +/-   ##
=======================================
  Coverage   55.66%   55.66%           
=======================================
  Files         331      331           
  Lines       26875    26875           
  Branches      453      453           
=======================================
  Hits        14959    14959           
  Misses      11911    11911           
  Partials        5        5           

Flag	Coverage Δ
error-stack	82.56% <0.00%> (ø)

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