This environment variable "R_TESTS" is set during testing, and it points to a file called Startup.Rs that is placed in the .libPaths(). If the .libPaths() is changed during the testing, then that file will not be found, and install.packages will fail to install a package with an error of source file not found. See: https://github.com/HenrikBengtsson/startup/issues/19.
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