esptool: fix elf2image conversion with "--dont-append-digest"

Append elf2image args so that "--dont-append-digest" do not override earlier
flash settings.

This was observed in case of ESP32-C2 image build where image build was failing
with `CONFIG_ESPTOOLPY_FLASHSIZE_DETECT`.

Related from earlier commit: 9c46c98b4c81699209fc9f44e3a3b1d94c5bf5b7

components/esptool_py/project_include.cmake

@@ -66,7 +66,7 @@ if(CONFIG_ESPTOOLPY_FLASHSIZE_DETECT)
     # Flash size detection updates the image header which would invalidate the appended
     # SHA256 digest. Therefore, a digest is not appended in that case.
     # This argument requires esptool>=4.1.
-    set(esptool_elf2image_args --dont-append-digest)
+    list(APPEND esptool_elf2image_args --dont-append-digest)
 endif()
 
 if(CONFIG_SECURE_SIGNED_APPS_RSA_SCHEME)