CI: Switch FreeBSD 15 to 15.0-ALPHA2

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Alexander Motin <alexander.motin@TrueNAS.com>
Closes #17749

.github/workflows/scripts/qemu-2-start.sh

@@ -121,7 +121,7 @@ case "$OS" in
     KSRC="$FREEBSD_SNAP/../amd64/$FreeBSD/src.txz"
     ;;
   freebsd15-0c)
-    FreeBSD="15.0-PRERELEASE"
+    FreeBSD="15.0-ALPHA2"
     OSNAME="FreeBSD $FreeBSD"
     OSv="freebsd14.0"
     URLxz="$FREEBSD_SNAP/$FreeBSD/amd64/Latest/FreeBSD-$FreeBSD-amd64-BASIC-CI-ufs.raw.xz"