buildhistory: avoid quoting issues with the layer list for build-id

buildhistory: avoid quoting issues with the layer list for build-id

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The layer list is multiline, and includes quotes, so including it within
quotes is problematic, particularly if the revision includes characters which
are not valid outside of a quoted string in shell. To reproduce this failure:
do a build with an scm layer not on a branch.

Signed-off-by: Christopher Larson <chris_larson@mentor.com (mailto:chris_larson@mentor.com)>