set correct mainnet.branch for node-only tests #2227

Job	Run time
forkless-update-data	0s
canary	0s
collator-selection	0s
forkless-update-no-data	0s
governance	0s
polkadot-types	0s
try-runtime	0s
xcm	0s
codestyle / rustfmt	13s
codestyle / clippy	2m 35s
codestyle / yarn_eslint	30s
unit-test / develop	14m 8s
node-only-update / execution matrix	27s
yarn-dev / opal	31m 31s
yarn-dev / sapphire	42m 11s
yarn-dev / quartz	41m 52s
yarn-dev / unique	27m 35s
node-only-update / opal	9h 22m 20s
node-only-update / sapphire	9h 17m 29s
node-only-update / quartz	9h 23m 44s
node-only-update / unique	9h 15m 2s
	1d 15h 59m 37s

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