Fix attaching usethis is lib path has changed

If the lib path has changed since loading usethis,
.onAttach potentially fails when it tries to attach
usethis as the new lib path might contain a different
usethis version.

Closes r-lib#1987.

R/zzz.R

@@ -68,5 +68,7 @@ devtools_default_options <- list(
 .onAttach <- function(libname, pkgname) {
   env <- as.environment(paste0("package:", pkgname))
   env[[".conflicts.OK"]] <- TRUE
-  suppressPackageStartupMessages((get("library", baseenv()))("usethis"))
+  usethis_lib <- dirname(getNamespaceInfo("usethis", "path"))
+  suppressPackageStartupMessages((
+    get("library", baseenv()))("usethis", lib.loc = usethis_lib))
 }