images/rpmrepo-snapshot: pin base image to fedora 40 #357

Job	Run time
Job Configuration	3s
Image Build/Test (cloud-tools-latest)	1h 10m 16s
Image Build/Test (kdc-latest)	7m 31s
Image Build/Test (koji-latest)	11m 52s
Image Build/Test (nfsd-latest)	24s
Image Build/Test (osbuild-ci-c10s-latest)	1m 28s
Image Build/Test (osbuild-ci-c9s-latest)	1m 38s
Image Build/Test (osbuild-ci-latest)	1m 44s
Image Build/Test (postgres-13-alpine)	41s
Image Build/Test (rpmrepo-ci-latest)	35s
Image Build/Test (rpmrepo-snapshot-latest)	39s
	1h 36m 51s

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