Fixed attribute handling in emergent constraint diagnostic for iris>=3.8

[schlunma]

Description

Since version 3.8, iris distinguishes explicitly between local and global attributes (see ESMValGroup/ESMValCore#2398), which causes an error in the emergent constraint diagnostics. This PR fixes that.

• Closes Diagnostic failure for recipe_schlund20esd.yml on v2.11.0rc1 #3604

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[valeriupredoi]

Thanks a lot, Manu (heroic PR off a train to Jena at 6am 🎖️ )