Only require unsafe-perm if you are root

Remove it from the docs, as the error message should be sufficient and
it seems weird to recommend unsafe-perm by default.

ci/build/npm-postinstall.sh

@@ -51,6 +51,18 @@ symlink_bin_script() {
   cd "$oldpwd"
 }
 
+command_exists() {
+  if [ ! "$1" ]; then return 1; fi
+  command -v "$@" > /dev/null
+}
+
+is_root() {
+  if command_exists id && [ "$(id -u)" = 0 ]; then
+    return 0
+  fi
+  return 1
+}
+
 OS="$(os)"
 
 main() {
@@ -75,17 +87,20 @@ main() {
     exit 1
   fi
 
-  case "${npm_config_user_agent-}" in npm*)
-    # We are running under npm.
-    if [ "${npm_config_unsafe_perm-}" != "true" ]; then
-      echo "Please pass --unsafe-perm to npm to install code-server"
-      echo "Otherwise the postinstall script does not have permissions to run"
-      echo "See https://docs.npmjs.com/misc/config#unsafe-perm"
-      echo "See https://stackoverflow.com/questions/49084929/npm-sudo-global-installation-unsafe-perm"
-      exit 1
-    fi
-    ;;
-  esac
+  # Under npm, if we are running as root, we need --unsafe-perm otherwise
+  # post-install scripts will not have sufficient permissions to do their thing.
+  if is_root; then
+    case "${npm_config_user_agent-}" in npm*)
+      if [ "${npm_config_unsafe_perm-}" != "true" ]; then
+        echo "Please pass --unsafe-perm to npm to install code-server"
+        echo "Otherwise post-install scripts will not have permissions to run"
+        echo "See https://docs.npmjs.com/misc/config#unsafe-perm"
+        echo "See https://stackoverflow.com/questions/49084929/npm-sudo-global-installation-unsafe-perm"
+        exit 1
+      fi
+      ;;
+    esac
+  fi
 
   if ! vscode_install; then
     echo "You may not have the required dependencies to build the native modules."