retry downloads a few times

prepare-opam.sh

@@ -5,6 +5,7 @@ export OPAMROOT="$(pwd)/opamroot"
 export OPAMJOBS="$((2*$CPU_CORES))"
 export OPAM_EDITOR="$(which false)"
 export OPAMDROPWORKINGDIR=1
+export OPAMRETRES=3
 
 # Delete OPAM 1.2 root, if that's what we got
 if test -d "$OPAMROOT" && fgrep 'opam-version: "1.2"' "$OPAMROOT/config" -q; then