fix: Default username in RoleSessionName

In case user does not have entry in `/etc/passwd` the `os.userInfo()`
call will throw `SystemError` exception as documented:
https://nodejs.org/docs/latest-v16.x/api/os.html#osuserinfooptions

Fixes aws#19401 issue.

It can be tested inside Docker for ad-hoc 1234 user ID:
```sh
docker run -u 1234 -e CDK_HOME=/tmp npm run cdk diff
```

The `CDK_HOME=/tmp` is a workaround for aws#7937 issue, where CDK complains
that it can't write cached info in user homedir, because it does not
exists.

Once aws#7937 will be fixed then aws#19401 will most likely hit users. However
above workaround is a viable option. Hence those two issues are related,
but not duplicated.

packages/aws-cdk/lib/api/aws-auth/sdk-provider.ts

@@ -459,7 +459,11 @@ function readIfPossible(filename: string): string | undefined {
  * @see https://docs.aws.amazon.com/STS/latest/APIReference/API_AssumeRole.html#API_AssumeRole_RequestParameters
  */
 function safeUsername() {
-  return os.userInfo().username.replace(/[^\w+=,.@-]/g, '@');
+  try {
+    return os.userInfo().username.replace(/[^\w+=,.@-]/g, '@');
+  } catch (e) {
+    return 'noname';
+  }
 }
 
 /**

packages/cdk-assets/lib/aws.ts

@@ -150,6 +150,10 @@ export class DefaultAwsClient implements IAws {
  * @see https://docs.aws.amazon.com/STS/latest/APIReference/API_AssumeRole.html#API_AssumeRole_RequestParameters
  */
 function safeUsername() {
-  return os.userInfo().username.replace(/[^\w+=,.@-]/g, '@');
+  try {
+    return os.userInfo().username.replace(/[^\w+=,.@-]/g, '@');
+  } catch (e) {
+    return 'noname';
+  }
 }