Further fixes to ci-main (#4221)

## Motivation and Context
Noticed `ci-main.yml` is [still
broken](https://github.com/smithy-lang/smithy-rs/actions/runs/16329860467/job/46129467122)
(CI won't run when a PR is merged to main)

## Description
Noticed from [the commit history on
main](https://github.com/smithy-lang/smithy-rs/commits/main/) that the
path `./.github/scripts/...` works, while
`./smithy-rs/.github/scripts/...` doesn't
([example](https://github.com/smithy-lang/smithy-rs/actions/runs/16308735604/job/46059996940)).

[The previous
PR](https://github.com/smithy-lang/smithy-rs/pull/4220/files#diff-a1a3cb9bdeb5541d148091d973cf266aa3b317e6415a86630e816cbe27cf8b9cR61)
used the latter path in `Tag and upload image` within `ci-main.yml`, but
it failed after being merged into main. This PR switches to
`./.github/scripts/...` to see if it works.

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