fix(setup): allow credentials to not be set if setup is disabled

[swiatekm-sumo]

Description

A recent change moved setup credentials to a temporary Secret, but failed to make this Secret conditional on setup being enabled. As a result, it's necessary to provide credentials even if setup is disabled, which is incorrect.

This PR doesn't include a test because our Helm template test framework doesn't allow not setting credentials in tests. I'll look into adding a test using Terratest instead in a separate PR.

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Checklist

• Changelog updated

[sumo-backporter]

The backport to release-v2 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release-v2 release-v2
# Navigate to the new working tree
cd .worktrees/backport-release-v2
# Create a new branch
git switch --create backport-2572-to-release-v2
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 c49c4073d224737e66cbfcc9e693791173196850
# Push it to GitHub
git push --set-upstream origin backport-2572-to-release-v2
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-v2

Then, create a pull request where the base branch is release-v2 and the compare/head branch is backport-2572-to-release-v2.