build: Support building Electron on msys2 (#29477)

Electron already seems to support `cygwin`, so `msys` is a natural
addition. This is the only required change as far as I can see on my
local development environment, as otherwise the build scripts don't
realize that msys = windows.

Notes: none
Signed-off-by: Juan Cruz Viotti <jv@jviotti.com>

Co-authored-by: Juan Cruz Viotti <jv@jviotti.com>

script/lib/config.py

@@ -18,6 +18,7 @@ BASE_URL = os.getenv('LIBCHROMIUMCONTENT_MIRROR') or \
 
 PLATFORM = {
   'cygwin': 'win32',
+  'msys': 'win32',
   'darwin': 'darwin',
   'linux': 'linux',
   'linux2': 'linux',