[ci] Fix msys2 build on AppVeyor

For whatever reason the env variables need to be the Windows way or they
end up being empty.

appveyor.yml

@@ -46,7 +46,7 @@ build_script:
   - 'if "%compiler%"=="msvc" nmake /f Makefile.vc CFG=%CFG% UNISCRIBE=1 DIRECTWRITE=1 install' # FREETYPE=1 install'
 
   - 'if "%compiler%"=="msys2" C:\msys64\usr\bin\bash -lc "pacman --noconfirm -S mingw-w64-$MSYS2_ARCH-{freetype,cairo,icu,gettext,gobject-introspection,gcc,gcc-libs,glib2,graphite2,pkg-config}"'
-  - 'if "%compiler%"=="msys2" C:\msys64\usr\bin\bash -lc "cd $APPVEYOR_BUILD_FOLDER; PATH=$PATH:/mingw64/bin:/mingw32/bin; ./autogen.sh --with-uniscribe --with-freetype --with-glib --with-gobject --with-cairo --with-icu --with-graphite2 --build=$MINGW_CHOST --host=$MINGW_CHOST --prefix=$MINGW_PREFIX; make; make check"'
+  - 'if "%compiler%"=="msys2" C:\msys64\usr\bin\bash -lc "cd $APPVEYOR_BUILD_FOLDER; PATH=$PATH:/mingw64/bin:/mingw32/bin; ./autogen.sh --with-uniscribe --with-freetype --with-glib --with-gobject --with-cairo --with-icu --with-graphite2 --build=%MINGW_CHOST% --host=%MINGW_CHOST% --prefix=%MINGW_PREFIX%; make; make check"'
 
 # disable automatic tests
 test: off