[One .NET] set env var automatically for $(UseInterpreter) (#6027)

Fixes: #6016
Context: https://github.com/dotnet/maui/blob/main/src/Templates/src/templates/maui-mobile/MauiApp1/Android/AndroidEnvironment.txt
Currently, the the .NET MAUI project template includes a
`@(AndroidEnvironment)` file for "Hot Reload" scenarios. Let's setup
this value by default so it will not be needed.

Author: jonathanpeppers

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.Android.Build.Tests/BuildTest.cs

@@ -3929,17 +3929,21 @@ public void AbiNameInIntermediateOutputPath ()
 publicvoidPackageNamingPolicy([Values("LowercaseMD5","LowercaseCrc64")]stringpackageNamingPolicy)
 {
 varproj=newXamarinAndroidApplicationProject();
+proj.SetProperty("UseInterpreter","true");
 proj.SetProperty("AndroidPackageNamingPolicy",packageNamingPolicy);
 proj.SetAndroidSupportedAbis("armeabi-v7a","x86");
 using(varb=CreateApkBuilder()){
 Assert.IsTrue(b.Build(proj),"build should have succeeded.");
 varenvironment=b.Output.GetIntermediaryPath(Path.Combine("__environment__.txt"));
 FileAssert.Exists(environment);
+varvalues=newList<string>{
+$"__XA_PACKAGE_NAMING_POLICY__={packageNamingPolicy}"
+};
 if(Builder.UseDotNet){
-Assert.AreEqual($"__XA_PACKAGE_NAMING_POLICY__={packageNamingPolicy}{Environment.NewLine}mono.enable_assembly_preload=0",File.ReadAllText(environment).Trim());
-}else{
-Assert.AreEqual($"__XA_PACKAGE_NAMING_POLICY__={packageNamingPolicy}",File.ReadAllText(environment).Trim());
+values.Add("mono.enable_assembly_preload=0");
+values.Add("DOTNET_MODIFIABLE_ASSEMBLIES=Debug");
 }
+Assert.AreEqual(string.Join(Environment.NewLine,values),File.ReadAllText(environment).Trim());
 }
 }
 

src/Xamarin.Android.Build.Tasks/Xamarin.Android.Common.targets

@@ -1513,6 +1513,7 @@ because xbuild doesn't support framework reference assemblies.
  <ItemGroup>
  <_GeneratedAndroidEnvironmentInclude="__XA_PACKAGE_NAMING_POLICY__=$(AndroidPackageNamingPolicy)" />
  <_GeneratedAndroidEnvironmentInclude="mono.enable_assembly_preload=0"Condition=" '$(AndroidEnablePreloadAssemblies)' != 'True' " />
+ <_GeneratedAndroidEnvironmentInclude="DOTNET_MODIFIABLE_ASSEMBLIES=Debug"Condition=" '$(UsingAndroidNETSdk)' == 'true' and '$(AndroidIncludeDebugSymbols)' == 'true' and '$(AndroidUseInterpreter)' == 'true' " />
  </ItemGroup>
  <WriteLinesToFile
 File="$(IntermediateOutputPath)__environment__.txt"