Migrate to Nix Flake

[lloeki]

What does this PR do?

Migrate from classic Nix to Nix Flake.

Motivation:

• flake.lock is much more potent than simply pinning pkgs by commit, like Gemfile.lock is for Ruby.
• With evaluation caching, flakes are so much faster.

Additional Notes:

Backwards compatibility with classic nix-build and nix-shell is provided via flake-compat shims.

How to test the change?

• nix flake show --all-systems
• nix develop

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-10-22 12:30:31

Comparing candidate commit aebdbd1 in PR branch lloeki/migrate-to-nix-flake with baseline commit 9a012ec in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 23 metrics, 2 unstable metrics.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 97.86%. Comparing base (1f1afc1) to head (aebdbd1).
Report is 192 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/datadog/core/telemetry/logging.rb	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4003      +/-   ##
==========================================
- Coverage   97.87%   97.86%   -0.01%     
==========================================
  Files        1313     1313              
  Lines       78352    78355       +3     
  Branches     3886     3886              
==========================================
+ Hits        76684    76686       +2     
- Misses       1668     1669       +1     

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