The `mingw-w64-libxml2` package brings its own `xmlcatalog.exe`. When
running e.g. `pacman -S docbook-xsl` while `MSYSTEM=MINGW64`, this
results in an `/etc/xml/catalog` that contains Windows-style paths.
Not what we want.
Let's make sure that we use the `xmlcatalog.exe` from the `libxml2`
package instead.
This is a companion to 2aefebb325 (docbook-{xml,xsl}: make sure to use
the _correct_ `xmlcatalog.exe`, 2019-11-08).
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
