ci: pin r-lib/actions/* in copilot-setup-steps.yml to @v2

[d-morrison]

Context

.github/workflows/copilot-setup-steps.yml currently pins r-lib's setup actions to @HEAD:

```yaml

• uses: r-lib/actions/setup-r@HEAD
• uses: r-lib/actions/setup-r-dependencies@HEAD
  ```

The repo is otherwise standardizing on @v2:

• .github/workflows/claude.yml already uses @v2 (PR #218).
• .github/workflows/R-CMD-check.yaml is migrating from @HEAD → @v2 as part of PR #207 (cran-submission).

Proposed change

After PR #207 lands, bump copilot-setup-steps.yml the same way:

```diff

• • uses: r-lib/actions/setup-r@HEAD

• • uses: r-lib/actions/setup-r@v2
    ...

• • uses: r-lib/actions/setup-r-dependencies@HEAD

• • uses: r-lib/actions/setup-r-dependencies@v2
    ```

Why this is low-priority

@HEAD works today; this is a cosmetic consistency fix, not a bug. Filing it so it doesn't get lost rather than bundling it into an unrelated PR. Sequencing after #207 avoids churning a file the larger PR doesn't touch.

Acceptance

• .github/workflows/copilot-setup-steps.yml uses r-lib/actions/setup-r@v2 and r-lib/actions/setup-r-dependencies@v2.
• Copilot Setup Steps workflow run succeeds on the change.