fix(kurl): do not overwrite encryption key when there is a server error

[emosbaugh]

What this PR does / why we need it:

Because pipefail is not set I believe the error is being ignored here and the encryption key is overwritten.

https://github.com/replicatedhq/kURL/blob/cfde7ad3e96062188ae53391717defbf6c669f38/scripts/common/kubernetes.sh#L514

For example:

$ asdf | base64 -d
zsh: command not found: asdf
$ echo $?         
0
$ secret_value=$(asdf)
zsh: command not found: asdf
$ echo $?             
127

Which issue(s) this PR fixes:

Does this PR require a test?

Does this PR require a release note?

Fixes an issue that can cause kURL installations to fail with an encryption key mismatch.

Does this PR require documentation?

[github-actions]

Testgrid Run(s) Executing @ https://testgrid.kurl.sh/run/KOTS-2025.12.22-674eaa-nightly-5735-merge-674eaa6-kotsadm-2025.12.22-674eaa-nightly-k8s-docker-2025-12-22T15:01:10Z