Fix "environent" typo in the Builder Protocol document. Remove "todo"
label from the nixVersion builtin because it appears to have been
implemented in 7fe1331570
Change-Id: I64aaa7cc93c8c6978cedd3bfacf756dacb4c7fc7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/13188
Tested-by: BuildkiteCI
Reviewed-by: flokli <flokli@flokli.de>