fix: envVar-identity-client-secret-not-passed

[hisImminence]

Which problem does the PR fix?

related camunda/camunda#25904

What's in this PR?

Using IDENTITY_CLIENT_SECRET envVar in case CAMUNDA_IDENTITY_CLIENT_SECRET not set.

Checklist

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[hisImminence]

@hamza-m-masood I just pushed our change. Could you take over (I could not run the golden file updates...)

[aabouzaid]

@hisImminence @hamza-m-masood, we probably need to do that for Alpha, too.

[hisImminence]

@hisImminence @hamza-m-masood, we probably need to do that for Alpha, too.

Good point!
--> #2703