Use the same keystore directory names as Lighthouse

Rationale: this makes moving keys between the clients eaiser

Other changes:

* Restore building with custom presets
  (defaultRuntimePreset is not a template in this mode)

beacon_chain/deposit_contract.nim

@@ -171,7 +171,7 @@ proc main() {.async.} =
   if cfg.cmd == StartUpCommand.generateSimulationDeposits:
     let
       walletData = WalletDataForDeposits(mnemonic: generateMnemonic(rng[]))
-      runtimePreset = defaultRuntimePreset()
+      runtimePreset = defaultRuntimePreset
 
     createDir(string cfg.outValidatorsDir)
     createDir(string cfg.outSecretsDir)

beacon_chain/keystore_management.nim

@@ -105,7 +105,7 @@ proc saveKeystore(rng: var BrHmacDrbgContext,
                   signingKey: ValidatorPrivKey, signingPubKey: ValidatorPubKey,
                   signingKeyPath: KeyPath): Result[void, KeystoreGenerationError] =
   let
-    keyName = $signingPubKey
+    keyName = "0x" & $signingPubKey
     validatorDir = validatorsDir / keyName
 
   if not existsDir(validatorDir):