Enable building with a dotnet not on PATH (#69186)

* Enable building with a dotnet not on PATH

In #68918 we removed inspecting DOTNET_HOST_PATH environment variable. This broke the scenario where a specific dotnet "hive" was installed to a location not on the PATH.

When the .NET SDK commands invoke a sub-process (for example MSBuild), it sets the DOTNET_HOST_PATH environment variable to tell the sub-process "this is where the dotnet.exe that invoked this command is located". See #21237 and dotnet/cli#7311 for more info.

This change reverts the behavior back to respect DOTNET_HOST_PATH, and if it isn't set it will just use "dotnet" and let the OS take care of finding the executable on the PATH.

Fix #69150

* Fix tests to workaround MSBuild searching the PATH itself.

* Respond to PR feedback.

Make the change smaller until @jaredpar gets back. Only make the minimal change required, which is to check DOTNET_HOST_PATH.

src/Compilers/Shared/RuntimeHostInfo.cs

@@ -49,16 +49,20 @@ internal static (string processFilePath, string commandLineArguments, string too
 
         internal static bool IsCoreClrRuntime => true;
 
+        private const string DotNetHostPathEnvironmentName = "DOTNET_HOST_PATH";
+
         /// <summary>
-        /// Get the path to the dotnet executable. In the case the host did not provide this information
-        /// in the environment this will return simply "dotnet".
+        /// Get the path to the dotnet executable. In the case the .NET SDK did not provide this information
+        /// in the environment this tries to find "dotnet" on the PATH. In the case it is not found,
+        /// this will return simply "dotnet".
         /// </summary>
-        /// <remarks>
-        /// See the following issue for rationale why only %PATH% is considered
-        /// https://github.com/dotnet/runtime/issues/88754
-        /// </remarks>
         internal static string GetDotNetPathOrDefault()
         {
+            if (Environment.GetEnvironmentVariable(DotNetHostPathEnvironmentName) is string pathToDotNet)
+            {
+                return pathToDotNet;
+            }
+
             var (fileName, sep) = PlatformInformation.IsWindows
                 ? ("dotnet.exe", ';')
                 : ("dotnet", ':');