Fix the demo_part_size not initialized issue when creating partition (#14296)

Why I did it
The demo_part_size should be initialized before creating partition.

How I did it
Move the initializing setting to the line before using it.

How to verify it

Author: xumia

installer/install.sh

@@ -138,6 +138,10 @@ fi
 # with "OS" or "DIAG".
 demo_type="%%DEMO_TYPE%%"
 
+# The build system prepares this script by replacing %%ONIE_IMAGE_PART_SIZE%%
+# with the partition size
+demo_part_size="%%ONIE_IMAGE_PART_SIZE%%"
+
 # The build system prepares this script by replacing %%IMAGE_VERSION%%
 # with git revision hash as a version identifier
 image_version="%%IMAGE_VERSION%%"
@@ -236,7 +240,6 @@ if [ "$install_env" = "onie" ]; then
     fi
 fi
 
-demo_part_size="%%ONIE_IMAGE_PART_SIZE%%"
 echo "ONIE_IMAGE_PART_SIZE=$demo_part_size"
 
 extra_cmdline_linux=%%EXTRA_CMDLINE_LINUX%%