more robust init: avoid spilling opam artifacts;

--- a/lib/Tools/ocaml_setup	Tue Mar 09 21:11:05 2021 +0100
+++ b/lib/Tools/ocaml_setup	Wed Mar 10 19:03:24 2021 +0100
@@ -13,6 +13,8 @@
 then
   isabelle_opam switch create -y "$ISABELLE_OCAML_VERSION"
 else
+  mkdir -p "$ISABELLE_OPAM_ROOT"
+  cd "$ISABELLE_OPAM_ROOT"
   isabelle_opam init -y --disable-sandboxing --no-setup --compiler="$ISABELLE_OCAML_VERSION"
 fi