chore: Declare ci_run flag in build.rs

[aborgna-q]

Rust 1.80 will require pre-declaring configuration flags that we want to set via RUSTFLAGS.
See the blog post.

Fixes #1067.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.55%. Comparing base (cc7f067) to head (96821bc).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1071      +/-   ##
==========================================
- Coverage   86.59%   86.55%   -0.04%     
==========================================
  Files          83       83              
  Lines       17547    17547              
  Branches    17547    17547              
==========================================
- Hits        15194    15188       -6     
- Misses       1533     1539       +6     
  Partials      820      820              

Flag	Coverage Δ
rust	86.55% <ø> (-0.04%)	⬇️

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[doug-q]

Conflicted on whether to be Glad or Sad about a build.rs...

[croyzor]

Can't this be defined in a Cargo.toml file?

[aborgna-q]

From the blogpost:

There is currently no way to expect a custom cfg other than with cargo::rustc-check-cfg in a build.rs.

Hopefully they add the option at some point