JS CI: save performance metrics

In !9922 (closed) I've noticed that we don't have previous performance metrics from the JS CI. E.g. see pipeline https://gitlab.haskell.org/ghc/ghc/-/jobs/1357671#L5151

This is despite some nightly jobs passing successfully: e.g. https://gitlab.haskell.org/ghc/ghc/-/jobs/1354095

We should look into why we don't have these metrics.

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