Use singleflight for caching environment.go calls

[oliveromahony]

Proposed changes

Use singleflight for the some of the environment calls in environment.go. This should speed up performance.

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

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[dhurley]

Can you also update the title of the PR?

[codecov-commenter]

Codecov Report

Attention: 56 lines in your changes are missing coverage. Please review.

Comparison is base (394a3fe) 66.28% compared to head (591a537) 66.12%.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #481      +/-   ##
==========================================
- Coverage   66.28%   66.12%   -0.16%     
==========================================
  Files         118      118              
  Lines       13382    13411      +29     
==========================================
- Hits         8870     8868       -2     
- Misses       3921     3950      +29     
- Partials      591      593       +2     

Files	Coverage Δ
src/core/nginx.go	43.81% <ø> (ø)
...inx/agent/v2/src/core/metrics/collectors/system.go	87.50% <100.00%> (-5.36%)	⬇️
...com/nginx/agent/v2/src/plugins/dataplane_status.go	67.75% <100.00%> (ø)
...om/nginx/agent/v2/src/core/metrics/sources/disk.go	69.56% <75.00%> (-2.44%)	⬇️
src/core/environment.go	51.02% <32.50%> (-1.51%)	⬇️

... and 1 file with indirect coverage changes

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