tdf#106854 svl windows: enable CNG by default

But keep the environment variable to disable it for testing purposes.
This means that signing with ECDSA keys works out of the box on Windows.

Change-Id: I2820deb45377c54b59cdbd27a490af9beaf62851
Reviewed-on: https://gerrit.libreoffice.org/56901
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>

svl/source/crypto/cryptosign.cxx

@@ -2422,8 +2422,8 @@ bool Signing::Verify(SvStream& rStream,
 
 bool isMSCng()
 {
-    static bool bMSCng = getenv("SVL_CRYPTO_CNG");
-    return bMSCng;
+    static bool bNoMSCng = getenv("SVL_CRYPTO_NOCNG");
+    return !bNoMSCng;
 }
 
 }