build: Use backend-agnostic meson commands for export

There's no alternative backend in sight for us, but it's nice to
only deal with a single build tool.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/182>

export-zips.sh

@@ -7,7 +7,7 @@ builddir=`mktemp -p $srcdir -d _build.XXXXXX` || exit 1
 installdir=`mktemp -p $srcdir -d _install.XXXXXX` || exit 1
 
 meson setup --prefix=$installdir -Dextension_set=all $srcdir $builddir
-ninja -C$builddir install
+meson install -C $builddir
 
 rm -rf $srcdir/zip-files
 mkdir $srcdir/zip-files