Add workaround for Mono xbuild MSBuild errors on macOS/Linux

- Skip MSBuildVersion parameter on non-Windows platforms to avoid triggering Mono's broken xbuild
- Add ignoreFailuresOnNonWindows option to NuGetRestore task
- Allow build to continue on macOS/Linux when MSBuild errors occur (Mono limitation)
- MSBuild errors are logged as warnings instead of failing the build

This allows NuGet restore to work on macOS/Linux where Mono's xbuild has known
limitations with NuGet restore operations.

Author: gamma

src/main/groovy/com/ullink/BaseNuGet.groovy

@@ -172,14 +172,38 @@ class BaseNuGet extends Exec {
             execAction.setExecutable(executable)
             execAction.setArgs(getArgs())
             execAction.setWorkingDir(project.projectDir)
-            execAction.execute()
+            
+            // Check if we should ignore failures on non-Windows (for Mono xbuild issues)
+            def shouldIgnoreFailures = false
+            if (this.hasProperty('ignoreFailuresOnNonWindows') && this.ignoreFailuresOnNonWindows) {
+                shouldIgnoreFailures = !isFamily(FAMILY_WINDOWS)
+            }
+            
+            try {
+                execAction.execute()
+            } catch (Exception e) {
+                if (shouldIgnoreFailures) {
+                    project.logger.warn("NuGet restore failed on non-Windows platform (likely Mono xbuild issue), ignoring: ${e.message}")
+                    return
+                }
+                throw e
+            }
         } else {
             // Fallback: try to call Exec.exec() via reflection
             try {
                 def execMethod = Exec.class.getDeclaredMethod("exec")
                 execMethod.setAccessible(true)
                 execMethod.invoke(this)
             } catch (Exception e) {
+                // Check if we should ignore failures on non-Windows
+                def shouldIgnoreFailures = false
+                if (this.hasProperty('ignoreFailuresOnNonWindows') && this.ignoreFailuresOnNonWindows) {
+                    shouldIgnoreFailures = !isFamily(FAMILY_WINDOWS)
+                }
+                if (shouldIgnoreFailures) {
+                    project.logger.warn("NuGet restore failed on non-Windows platform (likely Mono xbuild issue), ignoring: ${e.message}")
+                    return
+                }
                 throw new RuntimeException("Cannot execute NuGet command - no ExecActionFactory available", e)
             }
         }

src/main/groovy/com/ullink/NuGetRestore.groovy

@@ -7,6 +7,8 @@ import org.gradle.api.tasks.InputFile
 import org.gradle.api.tasks.Optional
 import org.gradle.api.tasks.OutputDirectory
 
+import static org.apache.tools.ant.taskdefs.condition.Os.*
+
 class NuGetRestore extends BaseNuGet {
 
     @Optional
@@ -36,6 +38,8 @@ class NuGetRestore extends BaseNuGet {
     @Optional
     @Input
     def packagesDirectory
+    @Input
+    def ignoreFailuresOnNonWindows = false
 
     NuGetRestore() {
         super('restore')
@@ -81,8 +85,15 @@ class NuGetRestore extends BaseNuGet {
         if (packagesDirectory) args '-PackagesDirectory', packagesDirectory
         if (solutionDirectory) args '-SolutionDirectory', solutionDirectory
         if (disableParallelProcessing) args '-DisableParallelProcessing'
-        if (!msBuildVersion) msBuildVersion = GradleHelper.getPropertyFromTask(project, 'version', 'msbuild')
-        if (msBuildVersion) args '-MsBuildVersion', msBuildVersion
+        
+        // Skip MSBuildVersion on non-Windows platforms (macOS/Linux) because Mono's xbuild/MSBuild
+        // doesn't work properly with NuGet restore and causes "Too many project files specified" errors
+        if (!isFamily(FAMILY_WINDOWS)) {
+            project.logger.debug("Skipping MSBuildVersion on non-Windows platform to avoid Mono xbuild issues")
+        } else {
+            if (!msBuildVersion) msBuildVersion = GradleHelper.getPropertyFromTask(project, 'version', 'msbuild')
+            if (msBuildVersion) args '-MsBuildVersion', msBuildVersion
+        }
 
         project.logger.info "Restoring NuGet packages " +
             (sources ? "from $sources" : '') +