MINOR: [R] Add the explicit mirror to install R dependencies

[HyukjinKwon]

Rationale for this change

Otherwise, it complains that the mirror has to be manually set in, e.g., R profile.
Should be best to make it self-contained.

What changes are included in this PR?

This PR fixes the R dev guides (https://arrow.apache.org/docs/4.0/r/articles/developing.html) to provide an explicit mirror when downloading related dependencies.

Are these changes tested?

Manually tested when setting up the dev.

Are there any user-facing changes?

No, dev-only.

[github-actions]

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See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview