ci: Install libpcre3-dev to fix pcre-light build failure

[kinyoklion]

Summary

The daily CI build has been failing consistently because the pcre-light Haskell package (a transitive dependency of launchdarkly-server-sdk) requires the libpcre C library, which is no longer pre-installed on the ubuntu-latest GitHub Actions runner image.

This adds a step to install libpcre3-dev before the build runs.

Review & Testing Checklist for Human

• Verify CI passes on this PR — this is the only real validation since the fix targets the CI runner environment
• Confirm no other system dependencies are missing (check the full CI build log for any additional failures beyond pcre-light)

Notes

• Devin Session
• Requested by: @kinyoklion

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