The default value for MICROPYPATH used in unix/main.c is
"~/.micropython/lib:/usr/lib/micropython" which has 2 problems when used in
the Windows port:
- it has a ':' as path separator but the port uses ';' so the entire string
is effectively discarded since it gets interpreted as a single path which
doesn't exist
- /usr/lib/micropython is not a valid path in a standard Windows
environment
Override the value with a suitable default.