gnu: r-rmarkdown: Ensure temporary files can be deleted.

* gnu/packages/statistics.scm (r-rmarkdown)[arguments]: Add phase
"copy-files-without-mode".

gnu/packages/statistics.scm

@@ -1953,6 +1953,20 @@ encoder/decoder, round-off-error-free sum and cumsum, etc.")
             "0c7gs9c8xdjfxviw0syh13pf3vys2b2ssixmnyqbji64xdscn7pz"))))
     (properties `((upstream-name . "rmarkdown")))
     (build-system r-build-system)
+    (arguments
+     `(#:phases
+       (modify-phases %standard-phases
+         ;; See https://github.com/rstudio/rmarkdown/pull/800
+         ;; The resource files are in the store and have mode 444.  After
+         ;; copying the files R fails to remove them again because it doesn't
+         ;; have write access to them.
+         (add-after 'unpack 'copy-files-without-mode
+           (lambda _
+             (substitute* "R/render.R"
+               (("file.copy\\(from = from," prefix)
+                (string-append prefix
+                               " copy.mode = FALSE, ")))
+             #t)))))
     (propagated-inputs
      `(("r-catools" ,r-catools)
        ("r-htmltools" ,r-htmltools)