universe: allow moving build logs to UNIVERSE_LOGDIR

This allows `make universe` or `make tinderbox`
to build from a read-only src tree.

Reviewed by:	ziaee, imp, delphij
Approved by:	lwhsu (mentor), emaste (mentor)
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D55566

share/man/man7/build.7

@@ -25,7 +25,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd April 28, 2026
+.Dd May 20, 2026
 .Dt BUILD 7
 .Os
 .Sh NAME
@@ -958,6 +958,11 @@ Only build worlds for each supported architecture.
 Only build kernels for each supported architecture.
 .It Va WITHOUT_KERNELS
 Only build worlds for each supported architecture.
+.It Va UNIVERSE_LOGDIR
+Write all build logs to this directory for each supported architecture.
+When invoking
+.Cm tinderbox ,
+also write the summary of all failed targets to this directory.
 .It Va UNIVERSE_TARGET
 Execute the specified
 .Xr make 1