focus on golden-path covering most scenarios

This should handle installation scenarios we can handle with
anything resembling confidence. Goal is approximating the existing
setup--not enforcing a best-practice...

Approaches (+ installer-handled, - manual) and configs each covers:

+ no change needed; /nix OK on boot volume:
  All pre-Catalina (regardless of T2 or FileVault use)

+ create new unencrypted volume:
  Catalina, pre-T2, no FileVault

+ create new encrypted-at-rest volume:
  Catalina, pre-T2, FileVault
  Catalina, T2, no FileVault

- require user to pre-create encrypted volume
  Catalina, T2, FileVault

scripts/install-nix-from-closure.sh

@@ -52,7 +52,7 @@ while [ $# -gt 0 ]; do
             NIX_INSTALLER_NO_CHANNEL_ADD=1;;
         --no-modify-profile)
             NIX_INSTALLER_NO_MODIFY_PROFILE=1;;
-        --create-volume)
+        --darwin-use-unencrypted-nix-store-volume)
             CREATE_DARWIN_VOLUME=1;;
         *)
             (
@@ -77,12 +77,13 @@ while [ $# -gt 0 ]; do
                 echo ""
             ) >&2
 
-            if [ "$(uname -s)" = "Darwin" ]; then
+            # darwin and Catalina+
+            if [ "$(uname -s)" = "Darwin" ] && [ "$macos_major" -gt 14 ]; then
                 (
-                    echo " --create-volume: Create an APFS volume for the store and create the /nix"
-                    echo "                  mountpoint for it using synthetic.conf."
-                    echo "                  (required on macOS >=10.15)"
-                    echo "                  See https://nixos.org/nix/manual/#sect-apfs-volume-installation"
+                    echo " --darwin-use-unencrypted-nix-store-volume: Create an APFS volume for the Nix"
+                    echo "              store and mount it at /nix. This is the recommended way to create"
+                    echo "              /nix with a read-only / on macOS >=10.15."
+                    echo "              See: https://nixos.org/nix/manual/#sect-macos-installation"
                     echo ""
                 ) >&2
             fi
@@ -98,12 +99,12 @@ if [ "$(uname -s)" = "Darwin" ]; then
     fi
 
     info=$(diskutil info -plist / | xpath "/plist/dict/key[text()='Writable']/following-sibling::true[1]" 2> /dev/null)
-    if ! [ -e $dest ] && [ -n "$info" ]; then
+    if ! [ -e $dest ] && [ -n "$info" ] && [ "$macos_major" -gt 14 ]; then
         (
             echo ""
             echo "Installing on macOS >=10.15 requires relocating the store to an apfs volume."
-            echo "Use sh <(curl https://nixos.org/nix/install) --create-volume or run the preparation steps manually."
-            echo "See https://nixos.org/nix/manual/#sect-apfs-volume-installation"
+            echo "Use sh <(curl https://nixos.org/nix/install) --darwin-use-unencrypted-nix-store-volume or run the preparation steps manually."
+            echo "See https://nixos.org/nix/manual/#sect-macos-installation"
             echo ""
         ) >&2
         exit 1