gnu: r-gtools: Make output deterministic.

* gnu/packages/statistics.scm (r-gtools)[arguments]: Add build phase to avoid
running tempdir at build time.

gnu/packages/statistics.scm

@@ -4042,6 +4042,14 @@ Zurich, including many that are related to graphics.")
         (base32
          "1xknwk9xlsj027pg0nwiizigcrsc84hdrig0jn0cgcyxj8dabdl6"))))
     (build-system r-build-system)
+    (arguments
+     `(#:phases
+       (modify-phases %standard-phases
+         (add-after 'unpack 'make-deterministic
+           (lambda _
+             (substitute* "R/checkReverseDependencies.R"
+               (("tempdir\\(\\)") "\"/tmp\""))
+             #t)))))
     (home-page "https://cran.r-project.org/web/packages/gtools")
     (synopsis "Various R programming tools")
     (description