GHA/non-native: bump to FreeBSD 15

[vszakats]

Follow-up to 34683b5 #20140

[testclutch]

Analysis of PR #20145 at 4466eecc:

Test 1516 failed, but it has been 1.2% flaky lately, so it's probably NOT a fault of the PR. Note that this test has failed in 2 different CI jobs (the link just goes to one of them).

Generated by Testclutch

[vszakats]

FreeBSD 15 jobs run significantly slower than 14.3. Same clang version, same cross-build Action.

e.g. 9.5m → 13m for the arm64 build:

https://github.com/curl/curl/actions/runs/20644568102/job/59280662169#logs
→
https://github.com/curl/curl/actions/runs/20644593056/job/59281084153?pr=20145#logs

[vszakats]

FreeBSD is 30% slower than 14.3. Very strange.

Déjà vu with Windows 2025 (compared to 2022). Also the latest OpenBSD upgrade broke things.
This makes it 3 platforms upgrades that are stalled by perf/reliability regressions.

https://github.com/curl/curl/actions/runs/20646897785/job/59285753522
https://github.com/curl/curl/actions/runs/20644593056/job/59281084174?pr=20145