Patch idea: an environment variable for setting the user ID

To R-devel,

In `R CMD build`, the ID of the user is automatically inserted into the
DESCRIPTION file, e.g.

Packaged: 2018-11-06 14:01:50 UTC; <MY USER ID>

This is problematic for those of us who work in corporate settings. We must
not divulge our user IDs in the packages we develop and release.

Jim Hester pointed out that these two lines in
`add_build_stamp_to_description_file()`
<https://github.com/wch/r-source/blob/521c90a175d67475b9f1b43d7ae68bc48062d8e6/src/library/tools/R/build.R#L170-L171>
are responsible. Could we consider his suggestion of using an optional
environment variable to overwrite the default behavior?

user <- Sys.getenv("R_BUILD_USERNAME")
if (!nzchar(user)) user <- Sys.info()["user"]
if(user == "unknown") user <- Sys.getenv("LOGNAME")

Will Landau