fix: try to resize build-time volume and copy nix store when done + test size before copy

[samrose]

trying to support this flow basically

mount /dev/xvdc /mnt/nix-temp

# Copy existing Nix installation to temp volume first
rsync -aHAXS /nix/ /mnt/nix-temp/

# Now bind mount over the original
mount --bind /mnt/nix-temp /nix

# Do all nix operations (they'll use the temp volume)
nix profile install ...

# Stop daemon before unmounting
systemctl stop nix-daemon || true

# Check size, unmount, and copy back
umount /nix
# /nix now shows the original pre-bind-mount content

# Clear it and copy from temp
rm -rf /nix/*
rsync -aHAXS /mnt/nix-temp/ /nix/

systemctl start nix-daemon || true

[samrose]

closing in favor of #1938