harden ACL installFIPS with explicit addon existence check

Adds a pre-flight check that /boot/acl/uki-addons/fips.addon.efi exists
before attempting to install it, producing a clear error message if the
source addon is missing. The Packer invocation already runs the script
under bash -eux so a missing addon would fail the build either way, but
the explicit check yields a much clearer diagnostic than the raw
'install: cannot stat' stderr.

Author: hbeberman

vhdbuilder/scripts/linux/acl/tool_installs_acl.sh

@@ -32,9 +32,15 @@ disableNtpAndTimesyncdInstallChrony() {
 installFIPS() {
     echo "Installing FIPS..."
 
-    install -D -m 0644 \
-     /boot/acl/uki-addons/fips.addon.efi \
-     /boot/EFI/Linux/acl.efi.extra.d/fips.addon.efi
+    local fips_addon_src="/boot/acl/uki-addons/fips.addon.efi"
+    local fips_addon_dst="/boot/EFI/Linux/acl.efi.extra.d/fips.addon.efi"
+
+    if [ ! -f "${fips_addon_src}" ]; then
+        echo "FIPS addon not found at ${fips_addon_src}" >&2
+        exit 1
+    fi
+
+    install -D -m 0644 "${fips_addon_src}" "${fips_addon_dst}"
 
     touch /etc/system-fips
     chmod 644 /etc/system-fips