For #26478
## Details
This PR updates the db snapshot script to have it add a `drop database
if exists` line to the top. This means that snapshots will start from a
clean state, so that they won't get out of sync with migrations.
## Testing
I tested this by:
1. starting with an up-to-date database, restoring an old snapshot while
on the main code branch, then trying `fleet prepare db`.
2. This failed because the migrations it tried to run were creating
tables that already existed in the database.
3. I made a new copy of this snapshot by dropping my db and doing
`create database fleet`, restoring the snapshot and snapshotting again.
4. On the main branch, I reset my db using `make db-reset` and then
restore my new snapshot
5. Finally, did `fleet prepare db` and this time it worked since the new
snapshot dropped the database before restoring.
Scott Gress
f5f2a16867