fix xpath and conditional bugs; xpath -> xmllint

- xpath -> xmllint: xpath's cli interface changed in Big Sur
  rather than add conditional logic for picking the correct
  syntax for xpath, I'm changing to xmllint --xpath, which
  appears to be consistent across versions I've tested...

- /plist/dict/key[text()='Writable']/following-sibling::true[1]
  doesn't do quite what's expected. It was written to try to
  select a <true /> node paired with the Writable key, but it
  will also select the *next* <true /> node that appears even
  if it was paired with another key.

- I think there's also a logic bug in the conditionals here.
  I'm not sure anyone ever actuall saw it, thanks to the xpath
  bug, though. With the xpath fix, this conditional passes if /nix
  does not exist, / IS writable, and the version is Catalina+.

  I think it meant to test for /nix does not exist, / is NOT
  writable, and the version is Catalina+. I reworked this lightly
  to make it a little clearer at the code level.

scripts/install-nix-from-closure.sh

@@ -110,8 +110,8 @@ if [ "$(uname -s)" = "Darwin" ]; then
         "$self/create-darwin-volume.sh"
     fi
 
-    info=$(diskutil info -plist / | xpath "/plist/dict/key[text()='Writable']/following-sibling::true[1]" 2> /dev/null)
-    if ! [ -e $dest ] && [ -n "$info" ] && { [ "$macos_major" -gt 10 ] || { [ "$macos_major" -eq 10 ] && [ "$macos_minor" -gt 14 ]; }; }; then
+    writable="$(diskutil info -plist / | xmllint --xpath "name(/plist/dict/key[text()='Writable']/following-sibling::*[1])" -)"
+    if ! [ -e $dest ] && [ "$writable" = "false" ]; then
         (
             echo ""
             echo "Installing on macOS >=10.15 requires relocating the store to an apfs volume."