fix(npm): allow npm constraint install to fail

lib/manager/npm/post-update/lerna.ts

@@ -60,7 +60,7 @@ export async function generateLockFiles(
       let installNpm = 'npm i -g npm';
       const npmCompatibility = config.constraints?.npm;
       if (validRange(npmCompatibility)) {
-        installNpm += `@${quote(npmCompatibility)}`;
+        installNpm += `@${quote(npmCompatibility)} || true`;
       }
       preCommands.push(installNpm, 'hash -d npm');
       cmdOptions = '--ignore-scripts  --no-audit';

lib/manager/npm/post-update/npm.ts

@@ -36,7 +36,7 @@ export async function generateLockFile(
     if (npmCompatibility) {
       // istanbul ignore else
       if (validRange(npmCompatibility)) {
-        installNpm = `npm i -g ${quote(`npm@${npmCompatibility}`)}`;
+        installNpm = `npm i -g ${quote(`npm@${npmCompatibility}`)} || true`;
       } else {
         logger.debug(
           { npmCompatibility },