GH Actions: tox-optional.yml: Remove misleading error messages regarding optional script packages

[mkoeppe]

For example on ubuntu-trusty-maximal (https://github.com/mkoeppe/sage/runs/2135025659),
configure correctly finds system perl_cpan_polymake_prereq
but then tests whether SPKG perl_cpan_polymake_prereq can be installed (it gives an error - by design).

This can be distracting in the GH Action logs.

Depends on #31163

CC: @kliem @jhpalmieri

Component: porting

Issue created by migration from https://trac.sagemath.org/ticket/31522

[mkoeppe]

comment:1

A possible fix would be through #31163

[mkoeppe]

comment:2

Sage development has entered the release candidate phase for 9.3. Setting a new milestone for this ticket based on a cursory review of ticket status, priority, and last modification date.

[mkoeppe]

comment:4

Taken care of by #31163

[mkoeppe]

Dependencies: #31163