Don't execute `can_sudo_without_pw()` until/unless it's absolutely necessary

[jennybc]

The next release of devtools uses pak::local_install_deps() for installing dependencies inside devtools::install(). I learned the hard way, i.e. via unsuccessful CRAN submission, that this can actually lead to a call to sudo.

I've now taken measures to ensure that devtools doesn't inadvertently trigger this, now or in the future. But it would also be nice if pak didn't call sudo -s id until/unless it has a proven need to know this information.

[jennybc]

I think it's computing this default that got me:

pak/src/library/pkgdepends/R/zzz-pkgdepends-config.R

Lines 39 to 43 in cda86c5

	default_sysreqs <- function(config) {
	plt <- config$get("sysreqs_platform")
	sysreqs_is_supported(plt) &&
	(is_root() || can_sudo_without_pw())
	}