fix: verify the metadata database certificate with sslmode=verify-full

- Switch every `metadata_db_sslmode` passed to bincraft in `.crow/` and `local/` from `require` to `verify-full`.
- Pass `sslrootcert = bincraft::postgres_sslrootcert("verify-full")` alongside `verify-full` in the scripts that call `DBI::dbConnect()` themselves, since libpq does not read the system trust store.
- Note in the README that the endpoint serves a Let's Encrypt certificate and how to verify it.

Part of rpkgs/bincraft#225.

local/build-one.R

@@ -115,7 +115,7 @@ for (ver in versions) {
     metadata_db_table = "single_builds",
     metadata_db_user = "rpkgs",
     metadata_db_password = Sys.getenv("PGPASS"),
-    metadata_db_sslmode = "require",
+    metadata_db_sslmode = "verify-full",
     metadata_db_port = 15432
   )
   if (sensitive) touched_minor <- TRUE else touched_generic <- TRUE