fix(): do not assign a non-string to process.env

Assigning a value to process.env[] leads to stringification of the
value. Example:

```
> process.env.test = {}
> process.env.test
'[object Object]'
```

This problem was hidden because `ConfigService.get` returns the value
of the validated config which would not be stringified.

A special case is when an environment variable has the value
`undefined`. This is because `ConfigService.get` falls back to
process.env which returns the stringified form `'undefined'`.

The stringification is deprecated and might result in a runtime error
in the future. See https://nodejs.org/api/process.html#processenv.

The argument has been changed to Record<string, unknown> to be more
type-safe. If this type was used in the original code, this bug could
have been avoided.

lib/config.module.ts

@@ -192,13 +192,18 @@ export class ConfigModule {
     return config;
   }
 
-  private static assignVariablesToProcess(config: Record<string, any>) {
+  private static assignVariablesToProcess(config: Record<string, unknown>) {
     if (!isObject(config)) {
       return;
     }
     const keys = Object.keys(config).filter(key => !(key in process.env));
     keys.forEach(
-      key => (process.env[key] = (config as Record<string, any>)[key]),
+      key => {
+        const value = config[key];
+        if (typeof value === 'string') {
+          process.env[key] = value;
+        }
+      },
     );
   }