unix/main: Add support for MICROPYINSPECT environment variable.

This adds support for a MICROPYINSPECT environment variable that works
exactly like PYTHONINSPECT; per CPython docs:

    If this is set to a non-empty string it is equivalent to specifying the
    -i option.

    This variable can also be modified by Python code using os.environ to
    force inspect mode on program termination.

ports/unix/main.c

@@ -647,6 +647,10 @@ MP_NOINLINE int main_(int argc, char **argv) {
         }
     }
 
+    const char *inspect_env = getenv("MICROPYINSPECT");
+    if (inspect_env && inspect_env[0] != '\0') {
+        inspect = true;
+    }
     if (ret == NOTHING_EXECUTED || inspect) {
         if (isatty(0)) {
             prompt_read_history();