[generator] Work around dotnet/roslyn#55564

Context: dotnet/roslyn#55564
Context: https://discord.com/channels/732297728826277939/732297837953679412/874959446747533323

.NET 6 Preview 7 introduced a C# compiler regression: use of
[C#6 string interpolation][0] with `StringBuilder.Append()` when
`$(LangVersion)` < 10.0 results in a CS8400 error:

	error CS8400: Feature 'interpolated string handlers' is not available in C# 8.0. Please use language version 10.0 or greater.

Because `azure-pipelines.yaml` specifies `DotNetCoreVersion: 6.0.x`,
our CI machines get auto-bumped to .NET 6 Preview 7, causing *all*
PR builds to now fail with this CS8400 failure.

There are two possible solutions:

 1. *Avoid* .NET 6 Preview 7.
 2. Remove select use of string interpolation.

(1) was initially done on dotnet#865, by using:

	DotNetCoreVersion: 6.0.100-preview.6.21355.2

However, it's not *all* use of string interpolation which triggers
the CS8400 error.  It's just use of string interpolation within
`StringBuilder.Append()` calls, in 3 locations.

Update `RegisterAttr.cs` to use a `(string)` cast with the existing
string interpolation use.  This works around the CS8400 error.

[0]: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/tokens/interpolated

tools/generator/SourceWriters/Attributes/RegisterAttr.cs

@@ -32,12 +32,12 @@ public override void WriteAttribute (CodeWriter writer)
 			var sb = new StringBuilder ();
 
 			if (UseGlobal)
-				sb.Append ($"[global::Android.Runtime.Register (\"{Name}\"");
+				sb.Append ((string) $"[global::Android.Runtime.Register (\"{Name}\"");
 			else
-				sb.Append ($"[Register (\"{Name}\"");
+				sb.Append ((string) $"[Register (\"{Name}\"");
 
 			if ((Signature.HasValue () || Connector.HasValue ()) && !UseShortForm)
-				sb.Append ($", \"{Signature}\", \"{Connector}\"");
+				sb.Append ((string) $", \"{Signature}\", \"{Connector}\"");
 
 			if (DoNotGenerateAcw && !AcwLast)
 				sb.Append (", DoNotGenerateAcw=true");