Bump stuff in nix shell

[lloeki]

What does this PR do?

• Bump nix channel
• Bump ruby to 3.2
• Add simple test in CI to ensure no breakage happens
• Introduce Nix derivation caching via cachix (this adds a secret to push to cachix)

Motivation

Keeping things up to date

Additional Notes

Derivations are cached publicly, therefore care must be taken not to store secrets as files, but that's already the current practice knowing that:

• it's a public repo with public CI
• we generate docker images that could capture the filesystem state and are published
• all secrets should be CI env vars

How to test the change?

• Manually

nix-shell
bundle install
# do stuff

• CI

[codecov-commenter]

Codecov Report

Merging #3013 (bf60e98) into master (386d499) will increase coverage by 0.00%. Report is 54 commits behind head on master. The diff coverage is 99.01%.

@@           Coverage Diff            @@
##           master    #3013    +/-   ##
========================================
  Coverage   98.08%   98.09%            
========================================
  Files        1316     1317     +1     
  Lines       73819    74362   +543     
  Branches     3385     3402    +17     
========================================
+ Hits        72403    72942   +539     
- Misses       1416     1420     +4     

Files Changed	Coverage Δ
lib/datadog/tracing/configuration/settings.rb	94.92% <ø> (-0.15%)	:arrow_down:
...atadog/tracing/contrib/active_record/events/sql.rb	97.61% <ø> (-0.16%)	:arrow_down:
...ng/contrib/active_support/cache/instrumentation.rb	86.20% <ø> (+0.49%)	:arrow_up:
lib/datadog/tracing/contrib/ethon/multi_patch.rb	97.91% <ø> (-0.09%)	:arrow_down:
lib/datadog/tracing/contrib/grpc/formatting.rb	98.21% <0.00%> (-1.79%)	:arrow_down:
lib/datadog/tracing/contrib/racecar/event.rb	97.43% <ø> (+2.31%)	:arrow_up:
lib/datadog/tracing/metadata/ext.rb	100.00% <ø> (ø)
...pec/datadog/tracing/configuration/settings_spec.rb	100.00% <ø> (ø)
...dog/tracing/contrib/active_record/multi_db_spec.rb	93.60% <ø> (-0.57%)	:arrow_down:
...tadog/tracing/contrib/active_record/tracer_spec.rb	97.59% <ø> (-0.14%)	:arrow_down:
... and 121 more

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[ivoanjo]

@lloeki is this still relevant?