Add support for handling OPAM_SWITCh env var.

prepare-opam.sh

@@ -17,7 +17,12 @@ if test -d "$OPAMROOT"; then
 else
     echo_color "$BOLDYELLOW" "[prepare-opam] Creating new opam root"
     mkdir "$OPAMROOT"
-    opam init --no-setup --disable-sandboxing -y
+    if [[ -z "${OPAM_SWITCH}" ]]; then
+        INITIAL_SWITCH="";
+    else
+        INITIAL_SWITCH="-c ${OPAM_SWITCH}";
+    fi;
+    opam init --no-setup --disable-sandboxing -y "${INITIAL_SWITCH}"
     FRESH_OPAM=yes
 fi
 eval `opam conf env`