Add support for building private dependencies in CI

[sjonpaulbrown]

PR Description: Add Support for Building Private Dependencies in CI

This PR introduces changes to support building private dependencies in the CI pipeline, specifically for private Cadence repositories.

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Key Changes

1. Update CI steps to load Cadence Deploy Key in CI:

   • Adds CI step to load the CADENCE_DEPLOY_KEY into the SSH agent for accessing private repositories.
   • Configures Git to use SSH for dependencies.
   • Dynamically sets SSH_AUTH_SOCK in the environment.
   • Warns if CADENCE_DEPLOY_KEY is not set, allowing the workflow to continue.

2. Update GOPRIVATE Configuration:

   • Updates GOPRIVATE to github.com/onflow/* in both the GitHub Actions workflow and the Makefile to bypass the Go module proxy for private repositories.

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Impact

These changes enable seamless fetching and building of private Cadence dependencies during CI, improving the reliability and security of the build process.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 41.54%. Comparing base (4162017) to head (337d345).

Additional details and impacted files

@@            Coverage Diff             @@
##            v0.37    #6728      +/-   ##
==========================================
- Coverage   41.55%   41.54%   -0.02%     
==========================================
  Files        2026     2026              
  Lines      181069   181069              
==========================================
- Hits        75248    75228      -20     
- Misses      99595    99615      +20     
  Partials     6226     6226              

Flag	Coverage Δ
unittests	41.54% <ø> (-0.02%)	⬇️

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