Environment variable keys should be case sensitive on unix

[JanProvaznik]

MSbuild treats environment variable keys as case insensitive where it should be OS-based, insensitive on windows, sensitive on unix-like

Probably needs changewave and a detection mechanism how to figure out if this change causes issues (logging in binary logger environment variable at time of use instead of at time of start of build, though that also risks logging secrets which was removed previously)