Free-threaded python3.13t interpreters are not discovered on Linux #569

Problem: Free-threaded interpreters that are installed only under a pythonX.Yt name are never discovered. Fedora's python3.13-freethreading package installs /usr/bin/python3.13t (Py_GIL_DISABLED=1, sys.prefix=/usr), and it does not appear in pet find or in the VS Code interpreter list. The executable-name patterns accept python3.13 but not the t suffix. pet resolve /usr/bin/python3.13t works.

Environment: Fedora 42 container, PET main @ 4ca7c03.

Before:

$ ls /usr/bin/python*
/usr/bin/python -> ./python3
/usr/bin/python3 -> python3.13
/usr/bin/python3.13
/usr/bin/python3.13t
/usr/bin/python3.13t-config
/usr/bin/python3.13t-x86_64-config
$ /usr/bin/python3.13t -c "import sys, sysconfig; print(sys.prefix, sysconfig.get_config_var('Py_GIL_DISABLED'))"
/usr 1
$ pet find --json   # summarized
  kind=LinuxGlobal version=3.13.13.final.0 exe=/usr/bin/python prefix=/usr
      symlinks: /usr/bin/python, /usr/bin/python3, /usr/bin/python3.13

Steps to verify fix:

1. In a Fedora 42 container, run dnf install -y python3 python3.13-freethreading.
2. Run pet find --json.
3. Verify /usr/bin/python3.13t is reported (LinuxGlobal, prefix /usr) in addition to /usr/bin/python.
4. Verify /usr/bin/python3.13t-config is not reported.
5. On Windows with the Python install manager's 3.14 and 3.14t, verify pet find lists the same environments as before.

Found by Copilot