Move Get/SetEnvironmentVariable to CoreLib

[JeremyKuhne]

Due to the lack of thread safety with environment variables on Unix the values are cached in the CoreFX implementation.

This causes Path.GetTempPath() and anything else that tries to get an environment variable in CoreLib to not have the up-to-date values. See the failing tests in dotnet/corefx#13920.

We should move the implementation to Internal.Runtime.Augments so we have one source of truth.

[JeremyKuhne]

@jkotas Any issues with doing this?

cc: @ianhays, @stephentoub

[jkotas]

Sounds reasonable to me.

[jkotas]

The Windows-only overloads that write to registry may still stay in corefx.

[AlexGhiondea]

@JeremyKuhne we marked this for 1.2 -- let me know if you disagree.

[danmoseley]

@JeremyKuhne does more remain here? I think you nentioned a change in the PAL.

[JeremyKuhne]

dotnet/corefx#14425