chore(scripts): use portable Bash shebangs

[eval-exec]

Summary

• resolve Bash through /usr/bin/env in all executable project shell scripts
• avoid assuming Bash is installed at /bin/bash
• preserve existing script behavior and executable modes

Testing

• bash -n scripts/copy-ui-files.sh scripts/download-ckb.sh scripts/package-for-test.sh scripts/release.sh
• verified no exact #!/bin/bash shebangs remain in the repository
• git diff --check origin/develop...HEAD

[eval-exec]

@chenyukang Ready to merge.

[Copilot]

Pull request overview

This PR updates executable project shell scripts to use a more portable Bash shebang (#!/usr/bin/env bash) instead of assuming Bash lives at /bin/bash, improving compatibility across environments while keeping existing script logic unchanged.

Changes:

• Replaced #!/bin/bash with #!/usr/bin/env bash in the project’s release/packaging helper scripts.
• Ensured scripts invoked directly in CI (e.g., ./scripts/download-ckb.sh ...) still have a valid Bash interpreter resolution via env.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
scripts/release.sh	Switch shebang to /usr/bin/env bash for portability.
scripts/package-for-test.sh	Switch shebang to /usr/bin/env bash for portability.
scripts/download-ckb.sh	Switch shebang to /usr/bin/env bash for portability.
scripts/copy-ui-files.sh	Switch shebang to /usr/bin/env bash for portability.

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