is_Cygwin: avoid `exec`ing anything

The `is_Cygwin` function is used, among other things, to determine
how executables are discovered in the `PATH` list by the `_which` function.

We are about to change the behavior of the `_which` function on Windows
(but not Cygwin): On Windows, we want it to ignore empty elements of the
`PATH` instead of treating them as referring to the current directory
(which is a "legacy feature" according to
https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03,
but apparently not explicitly deprecated, the POSIX documentation is
quite unclear on that even if the Cygwin project itself considers it to
be deprecated: https://github.com/cygwin/cygwin/commit/fc74dbf22f5c).

This is important because on Windows, `exec` does something very unsafe
by default (unless we're running a Cygwin version of Tcl, which follows
Unix semantics).

However, we try to `exec` something _inside_ `is_Cygwin` to determine
whether we're running within Cygwin or not, i.e. before we determined
whether we need to handle `PATH` specially or not. That's a Catch-22.

Therefore, and because it is much cleaner anyway, use the
`$::tcl_platform(os)` value which is guaranteed to start with `CYGWIN_`
when running a Cygwin variant of Tcl/Tk, instead of executing `cygpath
--windir`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>

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