GHA: speed up builds in torture jobs, tidy up

GHA: speed up builds in torture jobs, tidy up

- use cmake with ninja.
- drop unnecessary packages.

Also:
- Linux: switch to Ubuntu 20.24 runner. Initially for newer packages,
  then left it there for variation and as canary.
- Linux: make it easier to enable valgrind by dropping `-n` tflags.
- Linux: show `curl -V` after build.
- Linux: sync job names with macOS.
- Linux, macOS: review and adjust job timeouts.
- Linux, macOS: sync job configs more:
  Linux: enable libssh2, macOS: enable brotli, zstd, libssh2.
- macOS: prefer `source` (over manual `PATH` editing) for `venv` setup.
- macOS: drop redundant `-DENABLE_CURLDEBUG=ON`.
- macOS: drop redundant `-n` tflags. There is no valgrind on macOS.
- macOS: allow overriding test parallelism per job.
- macOS: unlock all disabled tests for torture jobs.

Speed-ups (configure/build stage):
- Linux !FTP: 83s -> 36s
  before: https://github.com/curl/curl/actions/runs/10948030198/job/30398134647
  after: https://github.com/curl/curl/actions/runs/10961987675/job/30440179978?pr=14972
- Linux FTP: 79s -> 32s
  before: https://github.com/curl/curl/actions/runs/10948030198/job/30398137070
  after: https://github.com/curl/curl/actions/runs/10961987675/job/30440182406?pr=14972
- macOS !FTP: 98s -> 29s
  before: https://github.com/curl/curl/actions/runs/10960141275/job/30434081412?pr=14972
  after: https://github.com/curl/curl/actions/runs/10960141275/job/30434082357?pr=14972
- macOS FTP: 129s -> 29s
  before: https://github.com/curl/curl/actions/runs/10960141275/job/30434082041?pr=14972
  after: https://github.com/curl/curl/actions/runs/10960141275/job/30434083050?pr=14972

Closes #14972