Don't prompt about using sudo if we're already root

doc/manual/src/installation/installing-binary.md

@@ -31,8 +31,8 @@ $ sh <(curl -L https://nixos.org/nix/install) --no-daemon
 ```
 
 This will perform a single-user installation of Nix, meaning that `/nix`
-is owned by the invoking user. You should run this under your usual user
-account, *not* as root. The script will invoke `sudo` to create `/nix`
+is owned by the invoking user. You can run this under your usual user
+account or root. The script will invoke `sudo` to create `/nix`
 if it doesn’t already exist. If you don’t have `sudo`, you should
 manually create `/nix` first as root, e.g.:
 
@@ -71,7 +71,7 @@ $ sh <(curl -L https://nixos.org/nix/install) --daemon
 
 The multi-user installation of Nix will create build users between the
 user IDs 30001 and 30032, and a group with the group ID 30000. You
-should run this under your usual user account, *not* as root. The script
+can run this under your usual user account or root. The script
 will invoke `sudo` as needed.
 
 > **Note**

scripts/install-multi-user.sh

@@ -337,10 +337,15 @@ __sudo() {
 _sudo() {
     local expl="$1"
     shift
-    if ! headless; then
+    if ! headless || is_root; then
         __sudo "$expl" "$*" >&2
     fi
+
+    if is_root; then
+        env "$@"
+    else
         sudo "$@"
+    fi
 }
 
 
@@ -891,17 +896,6 @@ EOF
 
 
 main() {
-    # TODO: I've moved this out of validate_starting_assumptions so we
-    # can fail faster in this case. Sourcing install-darwin... now runs
-    # `touch /` to detect Read-only root, but it could update times on
-    # pre-Catalina macOS if run as root user.
-    if is_root; then
-        failure <<EOF
-Please do not run this script with root privileges. I will call sudo
-when I need to.
-EOF
-    fi
-
     check_selinux
 
     if is_os_darwin; then
@@ -915,7 +909,10 @@ EOF
     fi
 
     welcome_to_nix
+
+    if ! is_root; then
         chat_about_sudo
+    fi
 
     cure_artifacts
     # TODO: there's a tension between cure and validate. I moved the