Get number of SNSes from SNS-WASM canister (#382)

# Motivation

When creating a snapshot, before killing the replica and archiving the
snapshot, we wait for the aggregator to scrap all SNSes.
Currently we specify to the aggregator how many SNSes it should expect.
This is not necessary because we can get this information from the
SNS-WASM canister.
And it can lead to unnecessary delays when the number is wrong.

# Changes

Instead of passing the number of expected SNSes to the
`dfx-sns-aggregator-wait` let the script determine the expected number
for itself by calling the SNS-WASM canister.

# Tested

CI passes and correctly detects 12 SNSes:

https://github.com/dfinity/snsdemo/actions/runs/10057983124/job/27800205191?pr=382
<img width="427" alt="image"
src="https://github.com/user-attachments/assets/6108d32e-f416-4ab0-845d-4876ea9c152a">

bin/dfx-snapshot-stock-make

@@ -45,7 +45,7 @@ dfx-stock-deploy --ic_commit "$DFX_IC_COMMIT" --ic_dir "$IC_REPO_DIR" --parallel
 
 : "Wait for aggregator to get all SNSs"
 # 2 SNSes were created by dfx-sns-demo.
-dfx-sns-aggregator-wait --num_sns "$((2 + PARALLEL_SNS_COUNT))"
+dfx-sns-aggregator-wait
 
 : "Wait for a checkpoint"
 dfx-network-wait-for-checkpoint --timeout 600

bin/dfx-sns-aggregator-wait

@@ -13,7 +13,7 @@ print_help() {
 # Source the clap.bash file ---------------------------------------------------
 source "$SOURCE_DIR/clap.bash"
 # Define options
-clap.define short=n long=num_sns desc="The number of SNSs to wait for" variable=EXPECTED_NUM_SNS default="1"
+# There are no custom options but we use clap for --help.
 # Source the output file ----------------------------------------------------------
 source "$(clap.build)"
 
@@ -75,6 +75,7 @@ get_sns_count() {
 }
 
 wait_for_sns_count() {
+  EXPECTED_NUM_SNS="$(dfx canister call nns-sns-wasm list_deployed_snses '(record{})' | idl2json | jq '.instances | length')"
   echo "Waiting for $EXPECTED_NUM_SNS SNSs"
   page_size=10
   required_page_count=$(((EXPECTED_NUM_SNS + page_size - 1) / page_size))