Fix KAMAL_DESTINATION missing during alias ERB render

[vulragrag-star]

Alias commands resolve through load_raw_config before create_from. Only create_from set ENV["KAMAL_DESTINATION"], so ERB that branches on that variable saw nil on the first render even when -d / --destination was on the command line.

This moves the env assignment into load_raw_config so alias resolution and normal config loading share the same behavior.

Fixes #1953

Test plan

• bundle exec ruby -Itest test/configuration_test.rb -n /destination/ (5 runs, 0 failures)
• Full test/configuration_test.rb (59 runs, 0 failures)
• ./bin/test on CI

[Copilot]

🟢 Approved

The shared loading path now matches existing configuration behavior and has focused regression coverage.

Pull request overview

Fixes #1953 by exposing the selected destination while ERB renders during alias resolution.

Changes:

• Moves KAMAL_DESTINATION assignment into shared raw-config loading.
• Adds regression coverage and destination-aware fixtures.

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File summaries

File	Description
lib/kamal/configuration.rb	Sets the destination environment variable before raw ERB rendering.
test/configuration_test.rb	Tests destination availability during raw loading.
test/fixtures/deploy_alias_needs_destination.yml	Provides an alias configuration requiring the destination.
test/fixtures/deploy_alias_needs_destination.staging.yml	Provides the destination-specific overlay.

Review details

• Files reviewed: 4/4 changed files
• Comments generated: 0
• Review effort level: Balanced

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