Add option to rez-build to preserve install directory on failure

Currently, rez-build will remove the artifacts in the installation directory on failure. There are some cases where this is undesirable. Can we add an option to not delete the contents in the installation directory when the build fails?

Start by locating the rez-build entry point and the failure path that removes artifacts from the installation directory. Define the new option there and verify that enabling it preserves the directory contents after a failed build while the default behavior remains unchanged.

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