Install libuv headers for R CI

[RAMitchell]

Summary

Fix R CI dependency installation by installing libuv1-dev in the Linux R test jobs.

Recent R CI failures on both this PR and master show the R package fs failing to configure because libuv is unavailable via pkg-config:

Package 'libuv', required by 'virtual:world', not found
Configuration failed because libuv was not found.
ERROR: configuration failed for package ‘fs’

The workflow already installs cmake, which previously allowed fs to build its bundled libuv, but the current dependency resolution now requires the system libuv development package. Installing libuv1-dev provides the headers and libuv.pc.

Changes

• Add libuv1-dev to the Ubuntu R test system dependencies.
• Add libuv1-dev to the Debian-container R test system dependencies.
• Keep cmake installed as a fallback for fs bundled-libuv builds.

Testing

• git diff --check
• Parsed .github/workflows/r_tests.yml with PyYAML.
• Commit hooks passed, including check yaml.

[Copilot]

Pull request overview

Updates the R CI workflow to install libuv development headers so the R package fs can configure successfully (via pkg-config) during Linux R test jobs.

Changes:

• Install libuv1-dev in the Ubuntu R test job.
• Install libuv1-dev in the Debian-container R test job.
• Adjust the cmake comment to reflect that it’s only needed if fs builds bundled libuv.

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