Allow for hardfork 26 on CI
new timestamp of hardfork is 1 September 2021 faketime timestamp is October 2021
Before this change faketime timestamp was used in system tests to setup node time, it was October. As hardfork is hardcoded in testnet fr 1 February, it didn't apply in some tests.
In this commit we specify hardfork time with environment variable for those tests. Alternatively or for future hardforks we could just regenerate block_log used to run system tests.