Fix relibc recipe

recipes/relibc/recipe.sh

-GIT=https://gitlab.redox-os.org/redox-os/relibc.git
-
-function recipe_build {
-    "$REDOX_MAKE" CARGO="env -u CARGO cargo" -j"$($NPROC)"
-    skip=1
-}
-
-function recipe_stage {
-    dest="$(realpath $1)"
-    "$REDOX_MAKE" CARGO="env -u CARGO cargo" DESTDIR="$dest" install
-    skip=1
-}

recipes/relibc/recipe.toml

-[recipe]
-# TODO: Set git link once userspace_fexec is merged.
-#GIT=https://gitlab.redox-os.org/redox-os/relibc.git
+[source]
+GIT=https://gitlab.redox-os.org/redox-os/relibc.git
 
 [build]
 template = "custom"