fix(@angular/cli): only show incompatible NPM error when NPM is used as package manager

(cherry picked from commit cae2209)

Author: alan-agius4

packages/angular/cli/commands/add-impl.ts

@@ -38,7 +38,7 @@ export class AddCommand extends SchematicCommand<AddCommandSchema> {
   }
 
   async run(options: AddCommandSchema & Arguments) {
-    ensureCompatibleNpm();
+    await ensureCompatibleNpm(this.context.root);
 
     if (!options.collection) {
       this.logger.fatal(

packages/angular/cli/commands/new-impl.ts

@@ -24,7 +24,7 @@ export class NewCommand extends SchematicCommand<NewCommandSchema> {
   }
 
   public async run(options: NewCommandSchema & Arguments) {
-    ensureCompatibleNpm();
+    await ensureCompatibleNpm(this.context.root);
 
     // Register the version of the CLI in the registry.
     const packageJson = require('../package.json');

packages/angular/cli/commands/update-impl.ts

@@ -254,7 +254,7 @@ export class UpdateCommand extends Command<UpdateCommandSchema> {
 
   // tslint:disable-next-line:no-big-function
   async run(options: UpdateCommandSchema & Arguments) {
-    ensureCompatibleNpm();
+    await ensureCompatibleNpm(this.context.root);
 
     // Check if the @angular-devkit/schematics package can be resolved from the workspace root
     // This works around issues with packages containing migrations that cannot directly depend on the package

packages/angular/cli/utilities/package-manager.ts

@@ -58,7 +58,11 @@ export async function getPackageManager(root: string): Promise<PackageManager> {
 /**
  * Checks if the npm version is version 6.x.  If not, display a message and exit.
  */
-export function ensureCompatibleNpm() {
+export async function ensureCompatibleNpm(root: string): Promise<void> {
+  if ((await getPackageManager(root)) !== PackageManager.Npm) {
+    return;
+  }
+
   try {
     const version = execSync('npm --version', {encoding: 'utf8', stdio: 'pipe'}).trim();
     const major = Number(version.match(/^(\d+)\./)?.[1]);