contrib: Fix NameError in signet miner gbt() #35044

The logging.warning call referenced bci["bestblockhash"], a variable from the calling scope do_generate() that is not available inside the Generate.gbt() method. This would crash with a NameError when getblocktemplate returned a template based on an unexpected previous block.

Use the bestblockhash parameter that was already being passed in and used correctly in the comparison on the line above.

The bug was introduced in 7b31332370 when the gbt logic was extracted into its own method — the if-condition was updated but the logging call was not.

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35044.

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ACK 701bc2dc02f73a10498ecf70c5d3523e0bb6b953

ACK 701bc2d

By reading the if statement and the warning message, it makes sense that we should output bestblockhash instead of bci["bestblockhash"] for two reasons

1. bci["bestblockhash"] is out of scope
2. in the if statement is checking for bestblockhash

Backported to 31.x in #35231.