Fix shadow warning

Summary:
The blockparams parameter of the lambda function shadows the
blockparams locale variable from FindEarliestAtLeast according to
GCC.

Test Plan:
With GCC as a compiler (tested with GCC 10.1):

cmake -GNInja .. -DCMAKE_CXX_FLAGS=Werror=shadow
ninja

Check the warning is gone.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6274