rustbuild: Skip some more non-relevant dist steps

This commit skips a few more dist tragets during compilation which shouldn't be
necessary.

* First, when packaging std we only take action when the host target is the
  build target. Otherwise we package the same artifacts a number of times, which
  shouldn't be necessary.
* Next, we apply the same logic to the save-analysis build. This is actually
  required for correctness as the build compiler is the only one which actually
  has save analysis information. This should fix an error seen on nightlies.

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