xmlto: Ensure macOS bash is used

Closes: https://trac.macports.org/ticket/71720

textproc/xmlto/Portfile

@@ -4,7 +4,7 @@ PortSystem          1.0
 
 name                xmlto
 version             0.0.29
-revision            1
+revision            2
 categories          textproc
 license             GPL-2+
 installs_libs       no
@@ -53,6 +53,7 @@ configure.env-append \
     GETOPT=${prefix}/bin/getopt \
     PAPER_CONF=${prefix}/bin/paperconf \
     XMLLINT=${prefix}/bin/xmllint \
+    XMLTO_BASH_PATH=/bin/bash \
     XSLTPROC=${prefix}/bin/xsltproc \
     FOP=${prefix}/bin/fop \
     W3M=${prefix}/bin/w3m