fix: use `build-for` when evaluating project variables

[mr-cal]

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As raised in Matrix (source), BUILD_FOR variables aren't evaluated correctly because the build-for arch was not passed to craft-parts.

Builds on #334, so review last 2 commits only.

Fixes https://github.com/canonical/snapcraft/issues/4770

[mr-cal]

Force-pushed to rebase from #334 onto hotfix/2.6