mingw: use MSYS2 quoting even when spawning shell scripts

mingw: use MSYS2 quoting even when spawning shell scripts

At the point where `mingw_spawn_fd()` is called, we already have a full
path to the script interpreter in that scenario, and we pass it in as
the executable to run, while the `argv` reflect what the script should
receive as command-line.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>