gnu: r-brms: Fix vignettes.

* gnu/packages/cran.scm (r-brms)[arguments]: Add phase 'set-HOME.

Change-Id: Ife0fcc2352b8bf47356c1684687fdf26b0281f6f

gnu/packages/cran.scm

@@ -37780,6 +37780,14 @@ inference diagnostics.
          "1gl1pdna356mfrf9816rqx8q7a3h3xzkn75bjd2zylgqw49ss88p"))))
     (properties `((upstream-name . "brms")))
     (build-system r-build-system)
+    (arguments
+     (list
+      #:phases
+      '(modify-phases %standard-phases
+         ;; When there is no HOME directory, this package will fail to find
+         ;; the vignette builder.
+         (add-after 'unpack 'set-HOME
+           (lambda _ (setenv "HOME" "/tmp"))))))
     (propagated-inputs
      (list r-abind
            r-backports