win: avoid duplicate separators in $PATH #12869

Seems like redundant env var separators (";" on Windows) in $PATH can
cause weird behavior. From #7377:

> After some time, system(['win32yank', '-o']) and system('win32yank -o')
> start returning different results: specifically first returns an
> empty string.
>
> 1. $PATH weirdly contains double semicolon followed by path to the
>    “installation directory” (unpacked directory from archive).
> 2. If I run `let $PATH=substitute($PATH, ';;', ';', 'g')` the problem is fixed.

close #7377
ref 224f99b85d

src/nvim/os/env.c

@@ -1176,7 +1176,9 @@ bool os_setenv_append_path(const char *fname)
       temp[0] = NUL;
     } else {
       xstrlcpy(temp, path, newlen);
-      xstrlcat(temp, ENV_SEPSTR, newlen);
+      if (ENV_SEPCHAR != path[pathlen - 1]) {
+        xstrlcat(temp, ENV_SEPSTR, newlen);
+      }
     }
     xstrlcat(temp, os_buf, newlen);
     os_setenv("PATH", temp, 1);