Improve signing performance in case of several internal keys

Instead of structuring the sign_transaction and (musig2 pubnonces)
functions as iterating:

  for all internal keys:
    for all inputs:
      <do stuff>

we reverse the nested loop to:

  for all inputs
    for all internal keys:
      <do stuff>

This avoids processing all the psbt keys for the inputs once for
every key placeholder (running it instead once per input), with
no functional change.