[Build] Increase the size of the installer image (#11869)

#### Why I did it
Fix the build failure caused by the installer image size too small. The installer image is only used during the build, not impact the final images.
See https://dev.azure.com/mssonic/build/_build/results?buildId=139926&view=logs&j=cef3d8a9-152e-5193-620b-567dc18af272&t=359769c4-8b5e-5976-a793-85da132e0a6f

```
+ fallocate -l 2048M ./sonic-installer.img
+ mkfs.vfat ./sonic-installer.img
mkfs.fat 4.2 (2021-01-31)
++ mktemp -d
+ tmpdir=/tmp/tmp.TqdDSc00Cn
+ mount -o loop ./sonic-installer.img /tmp/tmp.TqdDSc00Cn
+ cp target/sonic-vs.bin /tmp/tmp.TqdDSc00Cn/onie-installer.bin
cp: error writing '/tmp/tmp.TqdDSc00Cn/onie-installer.bin': No space left on device
[  FAIL LOG END  ] [ target/sonic-vs.img.gz ]
```

#### How I did it
Increase the size from 2048M to 4096M.
Why not increase to 16G like qcow2 image?
The qcow2 supports the sparse disk, although a big disk size allocated, but it will not consume the real disk size. The falocate does not support the sparse disk. We do not want to allocate a very big disk, but no use at all. It will require more space to build.

scripts/build_kvm_image.sh

@@ -36,7 +36,7 @@ create_disk()
 
 prepare_installer_disk()
 {
-    fallocate -l 2048M $INSTALLER_DISK
+    fallocate -l 4096M $INSTALLER_DISK
 
     mkfs.vfat $INSTALLER_DISK