Report _R_CHECK_ env vars

[hadley]

This is useful to see what's happening, especially if you want to double check that you tools/check.env settings are working

[tanho63]

tackling for tidy-dev-day:

@gaborcsardi do you have a preferred implementation for this? Just looking at it for tidy dev day now. Seems like it should be added to do_check() and grep through Sys.getenv() here:

rcmdcheck/R/package.R

Lines 229 to 231 in 2244576

	# user supplied env vars take precedence
	if (length(env)) chkenv[names(env)] <- env

or here:

rcmdcheck/R/print.R

Lines 18 to 23 in 2244576

	if (header) {
	cat_head("R CMD check results", paste(x$package, x$version))
	cat_line("Duration: ", pretty_sec(x$duration))
	cat_line()
	}

?

[tanho63]

@hadley suggests that devtools::check() already prints environment variables

https://github.com/r-lib/devtools/blob/2aa51ef9b508d9ee760f3d94c64c6cc0eded99d9/R/check.R#L215-L218

and that we should move the main printing logic from devtools::check() to rcmdcheck::rcmdcheck() instead.