ci: openbsd: Increase RAM disk's size

Its size was ~3.8GB before, which sometimes was not enough. OpenBSD CI task
often were failing due to no space left on device. Increase the RAM disk size
to ~4.6 GB.

Author: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/CAN55FZ2XVVPJRJmGB2DsL3gOrOinWh=HWvj6GO1cHzJ=6LwTag@mail.gmail.com
Backpatch-through: 18, where openbsd was added to CI

src/tools/ci/gcp_ram_disk.sh

@@ -15,7 +15,12 @@ case "`uname`" in
     umount /dev/sd0j # unused /usr/obj partition
     printf "m j\n\n\nswap\nw\nq\n" | disklabel -E sd0
     swapon /dev/sd0j
-    mount -t mfs -o rw,noatime,nodev,-s=8000000 swap $CIRRUS_WORKING_DIR
+    # Remove the per-process data segment limit so that mount_mfs can allocate
+    # large memory filesystems. Without this, mount_mfs mmap() may fail with
+    # "Cannot allocate memory" if the requested size exceeds the current
+    # datasize limit.
+    ulimit -d unlimited
+    mount -t mfs -o rw,noatime,nodev,-s=10000000 swap $CIRRUS_WORKING_DIR
     ;;
 esac