Fix bugs involving roxygen2::roxygenise() attaching the target package to the search path

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roxygen2::roxygenise() has the not-so-obvious side-effect of attaching the package that is being roxygenized, and leaving it attached on exit, even if it wasn't attached before.

This caused warnings to be thrown during the building of README.Rmd. The warnings came from DataPackageR::package_build() and DataPackageR::document(), which use roxygen2::roxygenise() directly or indirectly via devtools::document(), respectively, and from install.packages() warning that the package was already attached.

In this PR:

• Fixed these bugs safely by ensuring the packages will be unloaded via on.exit() regardless of whether install = TRUE or install = FALSE.

• Added tests to verify the intended behavior. The tests error out on develop and pass when cherry-picked to this PR branch.

• Edited README.Rmd to not install the mtcars20 package multiple times, and rebuilt the relevant section of README.md.

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Also tested rebuilding a real data package I've been working with (Schief546). Works as expected.

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Added 1 more tweak, done updating.