ci: avoid reusing stale apt cache

[tueda]

This PR avoids reusing (possibly) stale apt cache by including the runner OS name and version in the cache key.
It also updates check.rb to rerun FORM so errors are shown if the first run fails.

See also: awalsh128/cache-apt-pkgs-action#187

[jodavies]

That's certainly not an expected failure...

Issue512_3 (fixes.frm:3104) FAILED
===============================================================================
==4827== Memcheck, a memory error detector
==4827== Copyright (C) 2002-2022, and GNU GPL'd, by Julian Seward et al.
==4827== Using Valgrind-3.22.0 and LibVEX; rerun with -h for copyright info
==4827== Command: /home/runner/work/form/form/sources/tvorm -w2 1.frm
==4827== 
TFORM 5.0.0 (Jan 27 2026, v5.0.0-3-gd4ecf2d) 2 workers  Run: Fri Feb 13 14:21:37 2026
    #-

Time =       0.12 sec    Generated terms =      52500
            test         Terms in output =      52500
                         Bytes used      =    1267016
Please increase SmallExtension setup parameter.
Program terminating in thread 2 at 1.frm Line 15 --> 
Terminate called from sort.c:2306 (AddPoly)

Edit: fine after a re-run. Maybe this is a (very rare) failure depending on which tform threads get which terms. The test is intentionally very tight on space in the small and smallextension, to make sure GarbHand works correctly.

[coveralls]

coverage: 58.03% (-0.01%) from 58.04%
when pulling e1b5bfc on tueda:pr/fix/ci/cache-apt
into 9b0d11c on form-dev:master.