scripted-diff: test: Use int.to_bytes over struct packing

-BEGIN VERIFY SCRIPT-
 sed -i --regexp-extended 's!struct.pack\(.<?B., (.*)\)!\1.to_bytes(1, "little")!g'             $( git grep -l struct.pack )
 sed -i --regexp-extended 's!struct.pack\(.<I., (.*)\)!\1.to_bytes(4, "little")!g'              $( git grep -l struct.pack )
 sed -i --regexp-extended 's!struct.pack\(.<H., (.*)\)!\1.to_bytes(2, "little")!g'              $( git grep -l struct.pack )
 sed -i --regexp-extended 's!struct.pack\(.<i., (.*)\)!\1.to_bytes(4, "little", signed=True)!g' $( git grep -l struct.pack )
 sed -i --regexp-extended 's!struct.pack\(.<q., (.*)\)!\1.to_bytes(8, "little", signed=True)!g' $( git grep -l struct.pack )
 sed -i --regexp-extended 's!struct.pack\(.>H., (.*)\)!\1.to_bytes(2, "big")!g'                 $( git grep -l struct.pack )
-END VERIFY SCRIPT-

contrib/signet/miner

@@ -52,10 +52,10 @@ def signet_txs(block, challenge):
     mroot = block.get_merkle_root(hashes)
 
     sd = b""
-    sd += struct.pack("<i", block.nVersion)
+    sd += block.nVersion.to_bytes(4, "little", signed=True)
     sd += ser_uint256(block.hashPrevBlock)
     sd += ser_uint256(mroot)
-    sd += struct.pack("<I", block.nTime)
+    sd += block.nTime.to_bytes(4, "little")
 
     to_spend = CTransaction()
     to_spend.nVersion = 0