[build.nims] Thoroughly test `-flto` for release builds

[github-actions[bot]]

[build.nims] Thoroughly test -flto for release builds

in various tests, it seems to be performing roughly 10-15% better

https://github.com/arturo-lang/arturo/blob/bc5bd6b2c8cb65042d1b80d140ce8d2c5431977a/build.nims#L76

        "nowebview"         : "-d:NOWEBVIEW",
        "profile"           : "-d:PROFILE --profiler:on --stackTrace:on",
        # TODO(build.nims) Thoroughly test `-flto` for release builds
        #  in various tests, it seems to be performing roughly 10-15% better
        #  labels: performance, benchmark, installer
        "release"           : "--passC:'-flto'",
        "safe"              : "-d:SAFE",
        "vcc"               : "",

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