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ci: pin rivet to v0.12.0 #278

Job	Run time
Linux host (GTK4 build)	4m 48s
Racket core (contract tests)	1m 46s
Contract preflight (i18n + release metadata)	4s
macOS host (x64 build)	10m 7s
macOS host (arm64 build)	5m 24s
Windows host (WinRT build)	4m 48s
	26m 57s