fix CI msvc build outside git bash

It was only by accident that git bash worked for CI basing on the
"real_pwd()" expression. Replace the dubious replacement pattern with
"cygpath", which is present on git bash, Cygwin, and msys2 alike. In
particular git bash uses msys2 internally.

I was able to confirm a working build under msys2 with a wrapper for
MSVC invocation via the Ninja generator.

CI/before_script.msvc.sh

@@ -194,7 +194,11 @@ download() {
 }
 
 real_pwd() {
-	pwd | sed "s,/\(.\),\1:,"
+	if type cygpath >/dev/null 2>&1; then
+		cygpath -am "$PWD"
+	else
+		pwd # not git bash, Cygwin or the like
+	fi
 }
 
 CMAKE_OPTS=""