Update rust recipe

recipes/rust/recipe.sh

 GIT=https://gitlab.redox-os.org/redox-os/rust.git
-BRANCH=redox-2019-11-25
+BRANCH=redox-2020-07-27
 BUILD_DEPENDS=(llvm)
 DEPENDS="gcc cargo"
 PREPARE_COPY=0
@@ -18,7 +18,7 @@ function recipe_build {
     config="$(realpath ../config.toml)"
     source="$(realpath ../source)"
     unset AR AS CC CXX LD NM OBJCOPY OBJDUMP RANLIB READELF STRIP
-    python "$source/x.py" dist --config "$config" --jobs $(nproc) --incremental
+    python3 "$source/x.py" dist --config "$config" --jobs $(nproc) --incremental
     skip=1
 }