Run SPM-generated shell scripts under bash, not /bin/sh

[lazerg]

Summary:

SPM setup scripts start with set -euo pipefail, but every one of them runs under /bin/sh. On a machine where /bin/sh isn't bash (e.g. dash), that fails immediately with set: Illegal option -o pipefail, so no SwiftPM build gets past the first build phase.

Two spots in generate-spm-xcodeproj.js:

• shellScriptPhase hardcodes shellPath: '/bin/sh' for every PBXShellScriptBuildPhase it builds (Sync SPM Autolinking, Embed React Native Flavored Frameworks, plugin-contributed phases), while their bodies are bash-only. Now sets shellPath: '/bin/bash', and refreshes it on an already-injected project too.
• The "Sync SPM Autolinking" scheme pre-action runs the same kind of script, but a scheme pre-action ignores a build phase's shellPath entirely — Xcode reads the shell from the ActionContent's own shellToInvoke attribute instead (default /bin/sh). Now sets shellToInvoke = "/bin/bash" on both scheme creation and when adding/refreshing the pre-action on an existing scheme.

Fixes #58359

Changelog:

[IOS] [FIXED] - SwiftPM: run generated build scripts under bash instead of /bin/sh, so they don't break on a host where /bin/sh isn't bash

Test Plan:

yarn test packages/react-native/scripts/spm — 838 passed, including new cases for the refreshed shellPath/shellToInvoke on a project injected before this fix, and a scheme injected before the attribute existed.