Fix pvmfw.img signing error

This commit fixes the following error, by extracting
'SYSTEM/apex/com.android.virt.apex' from a target-files
zip file to a temp file, then gets the Microdroid AVB
key from it.

FileNotFoundError:
  [Errno 2] No such file or directory: 'SYSTEM/apex/com.android.virt.apex'

Bug: 384813199
Bug: 395932577
Test: m sign_target_files_apks
Test: sign_target_files_apks --allow_gsi_debug_sepolicy \
        --extra_apex_payload_key com.android.virt.apex= \
        -e com.android.virt.apex= \
        gsi_arm64-target_files-${build_id}.zip signed.zip
Test: unzip signed.zip IMAGES/pvmfw.img
Test: avbtool extract_public_key --key external/avb/test/data/testkey_rsa4096.pem --out key.pub
Test: grep -U -F -f key.pub IMAGES/pvmfw.img => grep: IMAGES/pvmfw.img: binary file matches
Change-Id: I4a4129d221778906fb739a574d7adb48e91e9eda