Shell-escape assignments to PATH in the top-level makefiles

Since 16fbf0191243e7c9dff6615b1424b5d39186b36c PATH is no longer set
to a hardcoded value on non-FreeBSD build hosts, so we can end up with
spaces in $PATH. Instead of only escaping PATH I updated all env PATH=
uses in the toplevel makefile. While many of these currently can't
contain any special characters (since the build would have failed
already), in theory this gets us closer to allowing build/source
directory to contain e.g. spaces.