Use NuGet user Actions variable

[0x5143]

Motivation

• The release and backplane publish workflows were referencing the NuGet username from secrets.NUGET_USER which caused an "Input required and not supplied: user" error when the repository uses an Actions variable for NUGET_USER instead of a secret.

Description

• Replace secrets.NUGET_USER with the Actions variable vars.NUGET_USER in the release workflow publish step by updating publish-nuget input to nuget-user: ${{ vars.NUGET_USER }} in ./.github/workflows/release.yml.
• Apply the same change to the Redis backplane publish workflow by updating the NuGet/login@v1 user input to user: ${{ vars.NUGET_USER }} in ./.github/workflows/publish-backplane-redis.yml.
• Ensure both workflow YAML files remain syntactically valid after the substitution.

Testing

• Verified both workflow files parse successfully via Ruby psych with ruby -e 'require "psych"; ARGV.each { |f| Psych.load_file(f); puts "OK #{f}" }' .github/workflows/release.yml .github/workflows/publish-backplane-redis.yml which returned OK for each file.
• Attempted YAML validation with Python yaml.safe_load but the environment lacks PyYAML, so that check was not performed (PyYAML unavailable).

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Codex Task