Revert "Revert "Break the cos-customizer dependency on snapshot.debian.org""

This reverts commit ff862d090297ddd17763ca533fe6ce94c14928d6.

This relands 2b99907, but installs dosfstools in cloudbuild.yaml,
coverage/cloudbuild.yaml, and run_unit_tests.sh.

BUG=b/293196753
TEST=`gcloud builds submit --config cloudbuild.yaml .`

Change-Id: I268e42d1ba9d9cabbf6f61caaa435ef17699d56d
Reviewed-on: https://cos-review.googlesource.com/c/cos/tools/+/57675
Cloud-Build: GCB Service account <228075978874@cloudbuild.gserviceaccount.com>
Reviewed-by: Arnav Kansal <rnv@google.com>
Tested-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Reviewed-by: Nandhini Rengaraj <nrengaraj@google.com>