rstudio: Fix recompilation issue with custom package set

This attempts to fix the issue described at
https://github.com/NixOS/nixpkgs/pull/22219#issuecomment-291801133.
Any change to the custom packages passed to RStudio causes this to
completely rebuild RStudio, which is completely unnecessary and also a
bit of a hindrance as it's a fairly slow build.

This rolls back most of that old PR, and instead implements something
more like rWrapper.  Existing configurations with the old useRPackages
will break.

pkgs/development/r-modules/wrapper.nix

@@ -1,19 +1,12 @@
 { stdenv, R, makeWrapper, recommendedPackages, packages }:
 
-stdenv.mkDerivation rec {
+stdenv.mkDerivation {
   name = R.name + "-wrapper";
 
   buildInputs = [makeWrapper R] ++ recommendedPackages ++ packages;
 
   unpackPhase = ":";
 
-  # This filename is used in 'installPhase', but needs to be
-  # referenced elsewhere.  This will be relative to this package's
-  # path.
-  passthru = {
-    fixLibsR = "fix_libs.R";
-  };
-  
   installPhase = ''
     mkdir -p $out/bin
     cd ${R}/bin
@@ -21,17 +14,6 @@ stdenv.mkDerivation rec {
       makeWrapper ${R}/bin/$exe $out/bin/$exe \
         --prefix "R_LIBS_SITE" ":" "$R_LIBS_SITE"
     done
-    # RStudio (and perhaps other packages) overrides the R_LIBS_SITE
-    # which the wrapper above applies, and as a result packages
-    # installed in the wrapper (as in the method described in
-    # https://nixos.org/nixpkgs/manual/#r-packages) aren't visible.
-    # The below turns R_LIBS_SITE into some R startup code which can
-    # correct this.
-    echo "# Autogenerated by wrapper.nix from R_LIBS_SITE" > $out/${passthru.fixLibsR}
-    echo -n ".libPaths(c(.libPaths(), \"" >> $out/${passthru.fixLibsR}
-    echo -n $R_LIBS_SITE | sed -e 's/:/", "/g' >> $out/${passthru.fixLibsR}
-    echo -n "\"))" >> $out/${passthru.fixLibsR}
-    echo >> $out/${passthru.fixLibsR}
   '';
 
   meta = {