MINOR refactor env var lookup for consistency with other repos (#2368)

Author: rayokota

src/Confluent.SchemaRegistry.Encryption.HcVault/HcVaultKmsClient.cs

@@ -21,11 +21,6 @@ public class HcVaultKmsClient : IKmsClient
 
         public HcVaultKmsClient(string kekId, string ns, string tokenId)
         {
-            if (tokenId == null)
-            {
-                tokenId = Environment.GetEnvironmentVariable("VAULT_TOKEN");
-                ns = Environment.GetEnvironmentVariable("VAULT_NAMESPACE");
-            }
             KekId = kekId;
             Namespace = ns;
             TokenId = tokenId;

src/Confluent.SchemaRegistry.Encryption.HcVault/HcVaultKmsDriver.cs

@@ -23,6 +23,11 @@ public IKmsClient NewKmsClient(IDictionary<string, string> config, string keyUrl
         {
             config.TryGetValue(TokenId, out string tokenId);
             config.TryGetValue(Namespace, out string ns);
+            if (tokenId == null)
+            {
+                tokenId = Environment.GetEnvironmentVariable("VAULT_TOKEN");
+                ns = Environment.GetEnvironmentVariable("VAULT_NAMESPACE");
+            }
             return new HcVaultKmsClient(keyUrl, ns, tokenId);
         }
     }

src/Confluent.SchemaRegistry.Encryption/LocalKmsClient.cs

@@ -16,14 +16,6 @@ public class LocalKmsClient : IKmsClient
 
         public LocalKmsClient(string secret)
         {
-            if (secret == null)
-            {
-                secret = Environment.GetEnvironmentVariable("LOCAL_SECRET");
-            }
-            if (secret == null)
-            {
-                throw new ArgumentNullException("Cannot load secret");
-            }
             Secret = secret;
             cryptor = new Cryptor(DekFormat.AES128_GCM);
             byte[] rawKey = Hkdf.DeriveKey(

src/Confluent.SchemaRegistry.Encryption/LocalKmsDriver.cs

@@ -22,6 +22,14 @@ public string GetKeyUrlPrefix()
         public IKmsClient NewKmsClient(IDictionary<string, string> config, string keyUrl)
         {
             config.TryGetValue(Secret, out string secret);
+            if (secret == null)
+            {
+                secret = Environment.GetEnvironmentVariable("LOCAL_SECRET");
+            }
+            if (secret == null)
+            {
+                throw new ArgumentNullException("Cannot load secret");
+            }
             return new LocalKmsClient(secret);
         }
     }