Move testnet ssh key (#5770)

* Factor out hardcoded testnet ssh key path

* Build/create test net ssh key path

* Rename testnet ssh dir

* Give testnetSSHDir a more generic name

* shellcheck

* favor hardcoded paths over `paths.sh`

* Put instance-startup-complete stamp in the scratch dir as well

* Rename `/solana` > `/solana-scratch`

net/scripts/add-testnet-solana-user-authorized_keys.sh

@@ -6,8 +6,6 @@ set -ex
 
 [[ -d /home/solana/.ssh ]] || exit 1
 
-
-
 if [[ -z $SOLANA_PUBKEYS ]]; then
   echo "Warning: source solana-user-authorized_keys.sh first"
 fi
@@ -15,9 +13,9 @@ fi
 # solana-user-authorized_keys.sh defines the public keys for users that should
 # automatically be granted access to ALL testnets
 for key in "${SOLANA_PUBKEYS[@]}"; do
-  echo "$key" >> /solana-authorized_keys
+  echo "$key" >> /solana-scratch/authorized_keys
 done
 
 sudo -u solana bash -c "
-  cat /solana-authorized_keys >> /home/solana/.ssh/authorized_keys
+  cat /solana-scratch/authorized_keys >> /home/solana/.ssh/authorized_keys
 "