Send R script R_HOME warning to stderr

Follow-up to #367. Redirecting the wasm R and Rscript to the host binaries
works, but R_HOME is no longer as expected. A warning about this is printed
to `stdout`, breaking subcommands and pipes when cross-compiling R packages
that run R or Rscript in e.g. Makevars.

This change redirects that warning to `stderr`.

See also https://bugs.r-project.org/show_bug.cgi?id=15070

patches/R-4.3.3/emscripten-r-home-stderr.diff

@@ -0,0 +1,13 @@
+Index: R-4.3.3/src/scripts/R.sh.in
+===================================================================
+--- R-4.3.3.orig/src/scripts/R.sh.in
++++ R-4.3.3/src/scripts/R.sh.in
+@@ -28,7 +28,7 @@ if test -n "${R_HOME}" && \
+
+ if test -n "${R_HOME}" && \
+    test "${R_HOME}" != "${R_HOME_DIR}"; then
+-  echo "WARNING: ignoring environment value of R_HOME"
++  echo "WARNING: ignoring environment value of R_HOME" >&2
+ fi
+ R_HOME="${R_HOME_DIR}"
+ export R_HOME

patches/R-4.3.3/series

@@ -12,6 +12,7 @@ emscripten-xhr-download.diff
 emscripten-disable-bcEval.diff
 emscripten-fix-blas-dlsym-search.diff
 emscripten-avoid-testing-issues.diff
+emscripten-r-home-stderr.diff
 fix-signrank-wilcox.diff
 fix-devUp.diff
 flang-avoid-maxloc.diff