[wasm] Run downlevel tests only on main

[maraf]

Based on #103118. Apparently the one test slipped through, but it should have the same limitation to run only latest TFM test on non-main branches.

Fixes #109653
Fixes #109627

Forwardport of #109723

[maraf]

Starting:    Wasi.Build.Tests (parallel test collections = on [2 threads], stop on fail = off)
    Wasi.Build.Tests.WasiTemplateTests.ConsoleBuildAndRunAOT(config: "Debug", singleFileBundle: False) [STARTING]
    Wasi.Build.Tests.SdkMissingTests.SimpleBuildDoesNotFail(config: "Debug") [STARTING]
    Wasi.Build.Tests.SdkMissingTests.SimpleBuildDoesNotFail(config: "Debug") [FAIL]
      System.IndexOutOfRangeException : Index was outside the bounds of the array.
      Stack Trace:
           at System.Collections.Generic.Dictionary`2.CopyEntries(Entry[] entries, Int32 count)
           at System.Collections.Generic.Dictionary`2.AddRange(IEnumerable`1 enumerable)
        /_/src/mono/wasi/Wasi.Build.Tests/BuildTestBase.cs(255,0): at Wasm.Build.Tests.BuildTestBase.BuildProject(BuildArgs buildArgs, String id, BuildProjectOptions options)
        /_/src/mono/wasi/Wasi.Build.Tests/SdkMissingTests.cs(70,0): at Wasi.Build.Tests.SdkMissingTests.BuildWithInvalidSdkPath(String config, String extraProperties, Boolean publish, Boolean expectSuccess)
        /_/src/mono/wasi/Wasi.Build.Tests/SdkMissingTests.cs(58,0): at Wasi.Build.Tests.SdkMissingTests.SimpleBuildDoesNotFail(String config)
           at System.RuntimeMethodHandle.InvokeMethod(Object target, Void** arguments, Signature sig, Boolean isConstructor)
           at System.Reflection.MethodBaseInvoker.InvokeDirectByRefWithFewArgs(Object obj, Span`1 copyOfArgs, BindingFlags invokeAttr)

Unrelated and weird. Trying re-run if it was some race condition