clone_gh_org_repos should respect branch field in packages.json

[remlapmot]

In https://github.com/MRCIEU/mrcieu.r-universe.dev/blob/main/packages.json we have to take quite a few repos from our forks (for some reason people write non-standard R packages in our field quite alot).

We send fixes from a branch on our fork. However this function isn't respecting the "branch" field, e.g.,

  {
    "package": "BWMR",
    "url": "https://github.com/remlapmot/BWMR",
    "branch": "suggestions"
  },

This adds that.

And also R CMD check says 3 non-standard files are found, so I added them to .Rbuildignore.

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.32%. Comparing base (66c4dc5) to head (08c4f8b).

Files with missing lines	Patch %	Lines
R/packages-json.R	33.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
+ Coverage   47.28%   47.32%   +0.03%     
==========================================
  Files          12       12              
  Lines        1510     1513       +3     
==========================================
+ Hits          714      716       +2     
- Misses        796      797       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.