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GH-42213: [Swift] Use "--warnings-as-errors" only on CI (#42214)

### Rationale for this change
Downstream clients are getting a conflicting options error when building with the "-warnings-as-errors" swift option. 

### What changes are included in this PR?
This PR disables the "-warnings-as-errors" swift options by default and enables them for CI builds only.  The Swift CI script will enable the options in the Package.swift files before running the build.

* GitHub Issue: #42213

Lead-authored-by: Alva Bandy <abandy@live.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>