There are a number of extensions, like the eps import,
that only become available when ps2pdf is available.
https://gitlab.com/inkscape/extensions/-/blob/master/eps_input.inx#L6
This is not so obvious, and this PR adds ghostscript (which provides ps2pdf)
explicitly so those extensions are always available and using a stable
version instead of relying on the PATH.
This will increase the inkscape closure by about 60MB,
which is quite a chunk, but perhaps not too bad on a
total of 1100MB.
4fb94610d9