ci: install libuv1-dev so R fs package builds on ubuntu-24.04 (#1420)

The `Install R language server` step started failing on the new
`ubuntu-24.04` runner image (`ubuntu-latest`) because the R `fs`
package falls back to a source build and its `configure` step calls
`pkg-config --exists libuv`, which fails with:

    Package libuv was not found in the pkg-config search path.
    fatal error: uv.h: No such file or directory
    ERROR: configuration failed for package 'fs'

That cascades through `pkgload` -> `roxygen2` -> `languageserver`,
making `test/solidlsp/r/test_r_basic.py` error out at setup with
`RuntimeError: R languageserver package is not installed`.

Verified locally on a clean `ubuntu:24.04` container: `libuv1-dev`
is not preinstalled but is available in the `noble` apt repo
(version `1.48.0-1.1build1`). Installing it makes
`pkg-config --exists libuv` succeed (`pkg-config --modversion libuv`
returns `1.48.0`), which is exactly what the `fs` configure script
needs.

Linux-only via `if: runner.os == 'Linux'` so the step is a no-op
on macOS and Windows runners (libuv is bundled differently there
and `fs` is delivered as a binary).

Refs: #1419

.github/workflows/pytest.yml

@@ -224,6 +224,10 @@ jobs:
         with:
           r-version: '4.4.2'
           use-public-rspm: true
+      - name: Install R sysdeps (libuv for fs package)
+        if: runner.os == 'Linux'
+        shell: bash
+        run: sudo apt-get update && sudo apt-get install -y libuv1-dev
       - name: Install R language server
         shell: bash
         run: |