[Helix] Need to stop permanently modifying path on windows

[HaoK]

Related to https://github.com/dotnet/core-eng/issues/10185

We made the paths get too long so dotnet-install stopped working on machines that don't get reimaged frequently, need to fixup runtests.cmd to stop permanently modifying the path

@pranavkm @MattGal

[HaoK]

Expected fix for this is to just switch to build in helix sdk support

[MattGal]

Is this work done?

[dougbu]

/cc @ryanbrandenburg

[HaoK]

Should be fixed by #24968 this time