std.os.getenv and std.os.getenvZ have nice compile errors when not linking
libc and using Windows.
std.os.getenvW is provided as a Windows-only API that does not require
an allocator. It uses the Process Environment Block.
std.process.getEnvVarOwned is improved to be a simple wrapper on top of
std.os.getenvW.
std.process.getEnvMap is improved to use the Process Environment Block
rather than calling GetEnvironmentVariableW.
std.zig.system.NativePaths uses process.getEnvVarOwned instead of
std.os.getenvZ, which works on Windows as well as POSIX.
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