Add a FreeBSD job on GitHub Actions to replace the retired Cirrus CI task

[neilpang]

Cirrus CI shut down on 2026-06-01. The last freebsd check-run here is on
603dacf (2026-01-02), and 23 commits have landed since with no FreeBSD
result -- so "Automated CI currently covers macOS, Windows, Ubuntu, Rocky
Linux, and FreeBSD" in the README, and the "FreeBSD 14" line under
Platforms, are no longer true.

This adds .github/workflows/build_freebsd.yml. It runs the same
.ci/unix-build.sh and .ci/unix-test.sh that freebsd_task ran, as a
non-root testuser created the same way, in a FreeBSD VM on a
GitHub-hosted runner. Only cmake is installed, as before.

Verified on my fork. FreeBSD 14.4, 3m49s for the whole job:

Tests run as user: testuser
100% tests passed, 0 tests failed out of 199
Total Test time (real) = 4.63 sec

https://github.com/neilpang/filesystem/actions/runs/33881441561/job/101050809350

release is '14' to match the Platforms list; adding '15' is a one-line
change if you want it.

One observation, not gated and not a regression: the std_filesystem_test
run that unix-test.sh already tolerates with "|| true" reports 4 failed
cases and 9 failed assertions against FreeBSD's libc++. I have not
classified them. Three of the nine:

CHECK( "///,foo,bar" == iterateResult(fs::path("///foo/bar")) )
with expansion: "///,foo,bar" == "/,foo,bar"

CHECK_THROWS_AS( fs::canonical(""), fs::filesystem_error )
because no exception was thrown where one was expected

CHECK( fs::canonical("", ec) == "" )
with expansion: "/home/runner/work/filesystem/filesystem/build" == ""

The rest are directory_entry::refresh(), assign("") and
replace_filename() not throwing where the standard asks. ghc::filesystem
itself is unaffected: ctest is 199/199.

The rockylinux8 and rockylinux9 tasks in .cirrus.yml are dead too. They
would run as containers on a GitHub runner; I left them alone here.