Ubuntu images for R 4.0 and higher should have checkbashisms script

[wch]

If a package has a configure or cleanup script, R CMD check --as-cran on R 4.0 needs the checkbashisms script to be available. For example:

* checking top-level files ... WARNING
##[warning]A complete check needs the 'checkbashisms' script.
See section ‘Configure and cleanup’ in the ‘Writing R Extensions’
manual.

https://github.com/r-lib/later/pull/121/checks?check_run_id=728590955#step:10:51

This can be installed with apt-get install devscripts. See rocker-org/rocker#365.

[github-actions]

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