Download page uses shasum from Perl, instead of GNU coreutils sha256sum

[Witold Baryluk]

Hi,

current download verify dialog shows this:

echo "96a8095001d447bbb9078925d72f7a77a3f62fbd78460093759af4394ce83d79
*ubuntu-19.10-desktop-amd64.iso" | shasum -a 256 --check

but the `shasum` is a program implemented in Perl, which might not be
installed. On my distro, I am able to have entire system installed
without any Perl at all, and no shasum program by default at all.

However, it is very likely to have GNU coreutils installed. It would
be actually hard not to have it installed.

So the dialog should show instead:

echo "96a8095001d447bbb9078925d72f7a77a3f62fbd78460093759af4394ce83d79
*ubuntu-19.10-desktop-amd64.iso" | sha256sum --check

I also done minor benchmarks, and sha256sum was about 6% faster. (Perl
shasum -a 256: 8.68s, coreutils sha256sum: 8.13s; real time).

The input / output format is exactly the same.

Cheers,
Witold