Skip "is installed" check if installing!

src/Squirrel/AppDesc.cs

@@ -262,6 +262,9 @@ public class AppDescWindows : AppDesc
 
         /// <inheritdoc />
         public override bool IsUpdateExe { get; }
+
+        /// <summary> True if Update.exe is currently performing first app install. </summary>
+        public bool IsInstalling { get; }
 
         /// <inheritdoc />
         public override SemanticVersion CurrentlyInstalledVersion { get; }
@@ -290,14 +293,15 @@ public AppDescWindows() : this(SquirrelRuntimeInfo.EntryExePath)
         /// Internal use only. Creates a AppDescWindows from the following rootAppDir and
         /// does not perform any path auto-detection.
         /// </summary>
-        internal AppDescWindows(string rootAppDir, string appId)
+        internal AppDescWindows(string rootAppDir, string appId, bool isInstalling = false)
         {
             AppId = appId;
             RootAppDir = rootAppDir;
             var updateExe = Path.Combine(rootAppDir, "Update.exe");
             UpdateExePath = updateExe;
             IsUpdateExe = Utility.FullPathEquals(updateExe, SquirrelRuntimeInfo.EntryExePath);
             CurrentlyInstalledVersion = GetLatestVersion()?.Version;
+            IsInstalling = isInstalling;
         }
 
         /// <summary>

src/Squirrel/UpdateManager.cs

@@ -219,10 +219,15 @@ private static IUpdateSource CreateSource(string urlOrPath, IFileDownloader urlD
                 return new SimpleFileSource(new DirectoryInfo(urlOrPath));
             }
         }
-
+        
         private Task<IDisposable> acquireUpdateLock()
         {
-            if (!ModeDetector.InUnitTestRunner() && _config.CurrentlyInstalledVersion == null)
+            bool installing = false;
+            if (SquirrelRuntimeInfo.IsWindows) {
+                installing = (_config as AppDescWindows)?.IsInstalling == true;
+            }
+
+            if (!installing && !ModeDetector.InUnitTestRunner() && _config.CurrentlyInstalledVersion == null)
                 throw new InvalidOperationException("Cannot perform this operation in a portable app (must be installed first).");
 
             lock (_lockobj) {

src/Update.Windows/Program.cs

@@ -254,7 +254,7 @@ static async Task Install(bool silentInstall, ProgressSource progressSource, str
                 .First().PackageName;
 
             var rootAppDir = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.LocalApplicationData), ourAppName);
-            var appDesc = new AppDescWindows(rootAppDir, ourAppName);
+            var appDesc = new AppDescWindows(rootAppDir, ourAppName, true);
             using var mgr = new UpdateManager(new SimpleFileSource(sourceDi), appDesc);
 
             Log.Info("About to install to: " + rootAppDir);