chore: use gettext-envsubst alpine package instead of workaround

[computator]

Proposed changes

As of Alpine v3.18 the envsubst command has been split out into it's own package (gettext-envsubst), so the workaround to extract it from the gettext package is no longer required.

https://pkgs.alpinelinux.org/packages?name=gettext-envsubst&branch=v3.18

Note

I just intended to create this PR as a quick edit while I was working on something else instead of creating a ticket. I see that there's more steps required for PRs and I can come back at some point and complete the rest of the steps, or someone else is welcome to reuse/complete this!

Checklist

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• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md and/or modules/README.md)

[github-actions]

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[computator]

I have hereby read the F5 CLA and agree to its terms