SL-18837: Use multi-line GitHub outputs to upload artifacts.

Having observed installer upload failures, I discovered the warning in
actions/upload-artifact/README.md about multiple concurrent jobs trying to
post the same pathname to the same artifact name. Try to disambiguate
artifacts not only for different platforms, but for different jobs running on
the same platform.

This change also reflects my understanding that an artifact is (effectively) a
distinct zip file that can contain multiple uploaded files. Because we'll want
to download metadata without having to download enormous installers, create a
separate metadata artifact per platform. Similarly, symbol files can get large:
use a third distinct artifact for symbol files.

But with those artifacts defined, leverage actions/upload-artifact's ability
to upload multiple paths to the same artifact. In build.sh, define bash arrays
installer, metadata, symbolfile and set up so that, on exit, each is written
to a GITHUB_OUTPUT variable with the corresponding name. This involves a
little magic to get macOS bash 3 to indirectly access an array.

These multi-line output variables are then used to drive the upload-artifact
step for each of the defined artifacts.

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