GHA: Minimally simplify FreeBSD job definitions

[kinke]

As build.d nowadays defines a proper default TARGET_FREEBSD<major> version if needed:

dmd/compiler/src/build.d

Lines 1387 to 1395 in f1b316b

	if (!userDflags.any!(f => f.startsWith("-version=TARGET_FREEBSD")))
	{
	// uname -K gives the kernel version, e.g. 1400097. The first two
	// digits correspond to the major version of the OS.
	immutable result = executeShell("uname -K");
	if (result.status != 0 || !result.output.take(2).all!isDigit())
	throw abortBuild("Failed to get the kernel version");
	dflags ~= ["-version=TARGET_FREEBSD" ~ result.output[0 .. 2]];
	}