allow '--toolchain' and '--toolchains'

as the function supports one or more arguments.

See discussion at
https://gitlab.gwdg.de/mpsd-cs/mpsd-software-environments/-/issues/1#note_799664

install-mpsd-software-environment.py

@@ -300,7 +300,8 @@ def main():
             subp.add_argument("release", type=str, help="Release version to install or remove")
             if cmd in ["install", "reinstall"]:
                 subp.add_argument(
-                    "--toolchains",
+                    "--toolchain",
+                    "--toolchains"
                     type=str,
                     nargs="+",
                     default="NONE",