WASI libc supports environment variables; update the README. (#172)

Author: sunfishcode

libc-top-half/README.md

@@ -13,7 +13,6 @@ Some major known missing areas include:
  - ipc
  - termios
  - nss
- - environment variables
  - timezones
  - non-builtin locales
  - TIOCGWINSZ (because cloudabi lacks it; affects isatty, line buffering for stdout)