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]

Thanks for opening a pull request!

If this is not a minor PR. Could you open an issue for this pull request on GitHub? https://github.com/apache/arrow/issues/new/choose

Opening GitHub issues ahead of time contributes to the Openness of the Apache Arrow project.

Then could you also rename the pull request title in the following format?

GH-${GITHUB_ISSUE_ID}: [${COMPONENT}] ${SUMMARY}

or

MINOR: [${COMPONENT}] ${SUMMARY}

See also:

• Other pull requests
• Contribution Guidelines - Contributing Overview

[jonkeane]

@github actions crossbow submit test-r-devdocs

[jonkeane]

I think we will want to pull out + add a bit of discussion explaining that this is really for someone who hasn't worked with R before and hasn't already set up their preferred repos.

[jonkeane]

We might want to pull this out into the text. For R users / developers they will have a preferred repo source configured already. For example, if someone is on linux + pointed at Posit Public Package manager for binaries, this will override that + be much slower for them.

I've also triggered a devdocs CI job to see if this impacts that too.

[HyukjinKwon]

Ah that's fine! I am fine with closing this too. I'll leave this to you 🫡

[jonkeane]

@github-actions crossbow submit test-r-devdocs

[github-actions]

Revision: 2476bc3

Submitted crossbow builds: ursacomputing/crossbow @ actions-c6e5649cbb

Task	Status
test-r-devdocs