fix: Evaluate RootLayout.ForCurrentDirectory lazily

This prevents start-up failures in the container environment (and
any other time we're not in a repo).

Fixes googleapis#14005

tools/Google.Cloud.Tools.ReleaseManager/CommandBase.cs

@@ -14,8 +14,10 @@
 
 using Google.Cloud.Tools.Common;
 using LibGit2Sharp;
+using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Threading;
 
 namespace Google.Cloud.Tools.ReleaseManager
 {
@@ -28,6 +30,10 @@ public abstract class CommandBase : ICommand
         // releases, but fundamentally it's "the current source of truth we're basing this release on".
         internal const string PrimaryBranch = "main";
 
+        // This is lazy so that we can construct instead of CommandBase even when the current directory isn't
+        // in a repo, but we don't go hunting for the root multiple times.
+        private readonly Lazy<RootLayout> _lazyLayout = new(RootLayout.ForCurrentDirectory, LazyThreadSafetyMode.ExecutionAndPublication);
+
         private readonly int _minArgs;
         private readonly int _maxArgs;
 
@@ -43,7 +49,7 @@ public abstract class CommandBase : ICommand
         /// <remarks>
         /// Currently this is always derived from the current directory, but we may later have a way of specifying it separately.
         /// </remarks>
-        protected RootLayout RootLayout { get; }
+        protected RootLayout RootLayout => _lazyLayout.Value;
 
         protected CommandBase(string command, string description, int minArgs, int maxArgs, string expectedArguments)
         {
@@ -52,7 +58,6 @@ protected CommandBase(string command, string description, int minArgs, int maxAr
             _minArgs = minArgs;
             _maxArgs = maxArgs;
             ExpectedArguments = expectedArguments;
-            RootLayout = RootLayout.ForCurrentDirectory();
         }
 
         protected CommandBase(string command, string description, params string[] argNames)