[wasm] fix weird build failures

`MSBuildSDKsPath` is set by runtime repo, and that interferes with the
test projects. To avoid this the `MSBuildSDKsPath` was set to `""` in
the test environment. But even that can negatively affect the build
because msbuild treats environment variables as "global properties" that
cannot be changed. This manifests when running:

`$ dotnet run --no-build`

.. it would fail with `/foo/bar.csproj is not a valid project file`.

Instead, explicitly *remove* `MSBuildSDKsPath` from the environment when
invoking the process.

src/mono/wasm/Wasm.Build.Tests/BuildEnvironment.cs

@@ -107,7 +107,6 @@ public BuildEnvironment()
             EnvVars["DOTNET_INSTALL_DIR"] = sdkForWorkloadPath;
             EnvVars["DOTNET_MULTILEVEL_LOOKUP"] = "0";
             EnvVars["DOTNET_SKIP_FIRST_TIME_EXPERIENCE"] = "1";
-            EnvVars["MSBuildSDKsPath"] = string.Empty;
             EnvVars["PATH"] = $"{sdkForWorkloadPath}{Path.PathSeparator}{Environment.GetEnvironmentVariable("PATH")}";
             EnvVars["EM_WORKAROUND_PYTHON_BUG_34780"] = "1";
 

src/mono/wasm/Wasm.Build.Tests/BuildTestBase.cs

@@ -840,6 +840,9 @@ public static (int exitCode, string buildOutput) RunProcess(string path,
                     processStartInfo.EnvironmentVariables[envVar.Key] = envVar.Value;
                     _testOutput.WriteLine($"\t{envVar.Key} = {envVar.Value}");
                 }
+
+                // runtime repo sets this, which interferes with the tests
+                processStartInfo.RemoveEnvironmentVariables("MSBuildSDKsPath");
             }
 
             Process process = new ();

src/mono/wasm/Wasm.Build.Tests/HelperExtensions.cs

@@ -120,5 +120,18 @@ public static void UpdateTo(this IDictionary<string, (string fullPath, bool unch
                 dict[filename] = (oldValue.fullPath, unchanged);
             }
         }
+
+        public static ProcessStartInfo RemoveEnvironmentVariables(this ProcessStartInfo psi, params string[] names)
+        {
+            var env = psi.Environment;
+            foreach (string name in names)
+            {
+                string? key = env.Keys.FirstOrDefault(k => string.Compare(k, name, StringComparison.OrdinalIgnoreCase) == 0);
+                if (key is not null)
+                    env.Remove("MSBuildSDKsPath");
+            }
+
+            return psi;
+        }
     }
 }

src/mono/wasm/Wasm.Build.Tests/ToolCommand.cs

@@ -161,6 +161,8 @@ private Process CreateProcess(string executable, string args)
             psi.Environment["DOTNET_MULTILEVEL_LOOKUP"] = "0";
             psi.Environment["DOTNET_SKIP_FIRST_TIME_EXPERIENCE"] = "1";
 
+            // runtime repo sets this, which interferes with the tests
+            psi.RemoveEnvironmentVariables("MSBuildSDKsPath");
             AddEnvironmentVariablesTo(psi);
             AddWorkingDirectoryTo(psi);
             var process = new Process
@@ -201,6 +203,7 @@ private void AddEnvironmentVariablesTo(ProcessStartInfo psi)
         {
             foreach (var item in Environment)
             {
+                _testOutput.WriteLine($"\t[{item.Key}] = {item.Value}");
                 psi.Environment[item.Key] = item.Value;
             }
         }