[ci] Allow R acceptance tests to run from a branch

[ebezzi]

Allow to run the R acceptance tests from a branch, using remotes::install_github. Useful with the current TileDBSOMA release process.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.12%. Comparing base (aa698f3) to head (7aeda51). Report is 29 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1147 +/- ## ======================================= Coverage 91.12% 91.12% ======================================= Files 77 77 Lines 5857 5892 +35 ======================================= + Hits 5337 5369 +32 - Misses 520 523 +3 ``` | [Flag](https://app.codecov.io/gh/chanzuckerberg/cellxgene-census/pull/1147/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=chanzuckerberg) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/chanzuckerberg/cellxgene-census/pull/1147/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=chanzuckerberg) | `91.12% <ø> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=chanzuckerberg#carryforward-flags-in-the-pull-request-comment) to find out more.

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[github-actions[bot]]

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