Fixtures: Make `pgtest` truly an optional dependency (#6502) #1562

Job	Run time
build-amd64 / linux/amd64	2m 22s
test-amd64 (aiida-core-base) / test	1m 36s
test-amd64 (aiida-core-dev) / test	2m 31s
test-amd64 (aiida-core-with-services) / test	1m 13s
build / linux/amd64,linux/arm64	8m 2s
publish-ghcr	1s
test-arm64	1s
publish-dockerhub	1s
	15m 47s

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