Fix NullableAttribute illink test failures (#90680)

When we started building with preview 7 in
5549f72, NullableAttribute in
these testcases started to use the attribute definition from the
framework, instead of generating it into the code. This broke the
`--used-attrs-only` optimization because `skip` assemblies (the
default for the framework in these testcases) are treated as if
all types in them are kept, for the purposes of the
`--used-attrs-only` optimization. (The optimization removes
attribute instances unless the attribute type is preserved for
some other reason).

It's not clear what the intended behavior of `--used-attrs-only`
is for `skip` assemblies, and the discussion in
dotnet/linker#952 indicates that it's
considered experimental, so this fixes the failures by using the
`link` action. This represents a more realistic scenario since
`skip` is mainly used in testing to avoid linking the framework
in every testcase.

Co-authored-by: Sven Boemer <sbomer@gmail.com>

src/tools/illink/test/Mono.Linker.Tests.Cases/Attributes/OnlyKeepUsed/NullableOnConstraints.cs

@@ -7,6 +7,7 @@ namespace Mono.Linker.Tests.Cases.Attributes.OnlyKeepUsed
 {
 	[SetupCSharpCompilerToUse ("csc")]
 	[SetupLinkerArgument ("--used-attrs-only", "true")]
+	[SetupLinkerTrimMode ("link")]
 	public class NullableOnConstraints
 	{
 		public static void Main ()

src/tools/illink/test/Mono.Linker.Tests.Cases/LinqExpressions/CanPreserveNullableCustomOperators.cs

@@ -5,6 +5,7 @@ namespace Mono.Linker.Tests.Cases.LinqExpressions
 {
 	[SetupCompileArgument ("/unsafe")]
 	[SetupLinkerArgument ("--used-attrs-only")]
+	[SetupLinkerTrimMode ("link")]
 	public class CanPreserveNullableCustomOperators
 	{
 		public static void Main ()