contrib: fix signet miner (sighash mismatch)

PSBT signing was changed to use SIGHASH_DEFAULT by default in #22514.
The signet miner script sets the sighash type of the created PSBT to
SIGHASH_ALL, hence this leads to a sighash mismatch when the
`walletprocesspsbt` RPC is called. Fix this by explicitly passing the
correct sighash type.

Note that the same change was needed in one of our functional tests,
see commit d3992669df.

Reported by gruve-p.

contrib/signet/miner

@@ -8,7 +8,7 @@ import base64
 import json
 import logging
 import math
-import os.path
+import os
 import re
 import struct
 import sys
@@ -493,10 +493,11 @@ def do_generate(args):
         logging.debug("Mining block delta=%s start=%s mine=%s", seconds_to_hms(mine_time-bestheader["time"]), mine_time, is_mine)
         mined_blocks += 1
         psbt = generate_psbt(tmpl, reward_spk, blocktime=mine_time)
-        psbt_signed = json.loads(args.bcli("-stdin", "walletprocesspsbt", input=psbt.encode('utf8')))
+        input_stream = os.linesep.join([psbt, "true", "ALL"]).encode('utf8')
+        psbt_signed = json.loads(args.bcli("-stdin", "walletprocesspsbt", input=input_stream))
         if not psbt_signed.get("complete",False):
             logging.debug("Generated PSBT: %s" % (psbt,))
-            sys.stderr.write("PSBT signing failed")
+            sys.stderr.write("PSBT signing failed\n")
             return 1
         block, signet_solution = do_decode_psbt(psbt_signed["psbt"])
         block = finish_block(block, signet_solution, args.grind_cmd)