Why Google Cloud Storage isn't taking my GOOGLE_APPLICATION_CREDENTIALS environment variable [HEROKU]?

I'm developing a POC website with Spring Boot to learn about GCP products, in specific Google Cloud Storage. The context is I am trying to save a profile picture when a user is registered.

From Google documentation, I can use

StorageOptions.getDefaultInstance().getService();

to validate my credentials if I have GOOGLE_APPLICATION_CREDENTIALS on my environment variables. The fact is, I do have GOOGLE_APPLICATION_CREDENTIALS as an environment variable (I'm using Linux Mint) pointing to my .json file (that is valid because using the path method it works), but it always returns 401 UNAUTHORIZED from GCP lib.

After spending HOURS searching, just to make sure I run this command on terminal:

gcloud auth application-default login

The response from this command is:

The environment variable [GOOGLE_APPLICATION_CREDENTIALS] is set to:
  [/home/<myuser>/<some_folder>/<myapplication>-<id>.json]
Credentials will still be generated to the default location:
  [/home/<myuser>/.config/gcloud/application_default_credentials.json]
To use these credentials, unset this environment variable before
running your application.

Do you want to continue (Y/n)?

After confirming and login with my browser, to my surprise it worked using StorageOptions.getDefaultInstance().getService();

So I'm assuming that GCP takes credentials from application_default_credentials file instead of the GOOGLE_APPLICATION_CREDENTIALS env var.

So on my local environment, it's working fine. The problem is: This website is deployed on Heroku and I can't use the path method because I don't want to put this credentials on GitHub.
I already put the environment variables at Heroku and didn't work.
Already tried to use these buildpacks too:

https://github.com/elishaterada/heroku-google-application-credentials-buildpack
https://github.com/gerywahyunugraha/heroku-google-application-credentials-buildpack

but I can't get it to work when on Heroku Cloud. Someone knows a way I can make this run using the 12factor rules, please?

In order to pass credentials you can do it via environment variables, that in fact is the one you are mentioning. To do it in Linux you should use the below command

export GOOGLE_APPLICATION_CREDENTIALS="[PATH]"

Nevertheless, something to keep in mind is that this variable only applies to your current shell session, so if you open a new session, set the variable again.

On the other hand, another option for perform this task is passing credentials using code, and as you can see in the shared link you should use something like below.

GoogleCredentials credentials = GoogleCredentials.fromStream(new FileInputStream(jsonPath))
        .createScoped(Lists.newArrayList("https://www.googleapis.com/auth/cloud-platform"));
  Storage storage = StorageOptions.newBuilder().setCredentials(credentials).build().getService();

Finally, in case you do not want to point the credentials inside of your applications's code, you can use Cloud Key Management Service. I think that KMS is a good option for your case but at the end, you should make the decision.