Work around Inkscape command-line breaking change.

Work around Inkscape command-line breaking change.

It looks as if Inkscape 1.0 changed the command-line syntax for
converting an SVG into a PNG, and didn't provide any syntax I can see
that works in both versions. Bodge around that with an initial run of
'inkscape --version', so that this package build should work both
before and after that breaking change.