refactor: Use static_cast<decltype(...)> to suppress integer sanitizer warning

This refactor does not change any behavior, except for the integer
sanitizer warning.

Can be tested via:

UBSAN_OPTIONS="suppressions=$(pwd)/test/sanitizer_suppressions/ubsan:print_stacktrace=1:halt_on_error=1:report_error_type=1" ./bld-cmake/bin/bitcoin-cli -stdinrpcpass uptime

test/sanitizer_suppressions/ubsan

@@ -55,7 +55,6 @@ unsigned-integer-overflow:TxConfirmStats::EstimateMedianVal
 unsigned-integer-overflow:InsecureRandomContext::rand64
 unsigned-integer-overflow:InsecureRandomContext::SplitMix64
 unsigned-integer-overflow:bitset_detail::PopCount
-implicit-integer-sign-change:SetStdinEcho
 implicit-integer-sign-change:compressor.h
 implicit-integer-sign-change:crypto/
 implicit-integer-sign-change:TxConfirmStats::removeTx