Cygwin: Do not show Error dialogs by default

Cygwin: Do not show Error dialogs by default

...in a non-Cygwin child process.  Backported from MSYS2.
Downstream commit message follows.

In https://github.com/msys2/msys2-runtime/pull/18, we discussed a change
that would allow default Windows error handling of spawned processes to
kick in (such as registered JIT debuggers). We even agreed that it would
make sense to hide this functionality behind a flag, `winjitdebug`.

However, when this got upstreamed as 21ec498d7f (cygwin: use
CREATE_DEFAULT_ERROR_MODE in spawn, 2020-12-09), that flag was deemed
unnecessary.

But it would appear that it _is_ necessary: As reported in
https://github.com/msys2/MSYS2-packages/pull/2414#issuecomment-810841296
this new behavior is pretty disruptive e.g. in CI scenarios.

So let's introduce that `winjitdebug` flag (settable via the environment
variable `MSYS`) at long last.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>