fix: increase default number of proving agents #6824

Job	Run time
setup / start-builder	11s
setup / setup	51s
noir-projects	13s
build	5m 30s
bb-bench-binaries	14s
bb-native-tests	12s
yarn-project-formatting	4m 1s
docs-preview	9m 12s
yarn-project-test	3m 46s
prover-client-test	8m 33s
setup-bench / start-builder	1m 10s
setup-bench / setup	54s
bb-bench	13s
e2e (bench-process-history)	17m 58s
e2e (bench-publish-rollup)	11m 29s
e2e (bench-tx-size)	5m 1s
e2e (e2e-browser)	4m 4s
e2e (e2e-docs-examples)	3m 40s
e2e (e2e-sandbox-example)	4m 13s
e2e (e2e-tests)	9m 43s
e2e (flakey-e2e-tests)	6m 38s
e2e (guides-dapp-testing)	5m 43s
e2e (guides-writing-an-account-contract)	3m 40s
e2e (integration-l1-publisher)	4m 31s
e2e (pxe)	3m 14s
e2e (uniswap-trade-on-l1-from-l2)	9m 1s
merge-check	1s
notify	0s
	2h 3m 56s

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