more work arounds for more opam quirks

prepare-opam.sh

 ## This script installs the build dependencies for CI builds.
 
-# Prepare OPAM configuration
+# Prepare OPAM configuration.
 export OPAMROOT="$(pwd)/opamroot"
 export OPAMJOBS="$((2*$CPU_CORES))"
-export OPAM_EDITOR="$(which false)"
+export OPAMEDITOR="$(which false)"
+export OPAMRETRIES=3
+# Work around some OPAM bugs/quirks.
 export OPAMDROPWORKINGDIR=1
-export OPAMRETRES=3
+export OPAMPRECISETRACKING=1
 
 # Delete OPAM 1.2 root, if that's what we got
 if test -d "$OPAMROOT" && fgrep 'opam-version: "1.2"' "$OPAMROOT/config" -q; then