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/missing-cran-packages-db.R

@@ -10,7 +10,8 @@ con <- DBI::dbConnect(
   port = 15432,
   user = "r_binaries",
   password = Sys.getenv("PGPASS"),
-  sslmode = "require"
+  sslmode = "verify-full",
+  sslrootcert = bincraft::postgres_sslrootcert("verify-full")
 )
 
 `%nin%` <- Negate(`%in%`)
@@ -45,12 +46,12 @@ data <- dbGetQuery(
   "SELECT name FROM single_builds WHERE platform = $1 AND arch = $2 and removed = FALSE;",
   params = list(platform, arch)
 )
-removed_pkgs = get_removed_cran_packages(lubridate::interval(
+removed_pkgs <- get_removed_cran_packages(lubridate::interval(
   lubridate::today(),
   lubridate::today() - 2
 ))$name
 pkgs_db <- unique(data$name)
-pkgs_db = setdiff(pkgs_db, removed_pkgs)
+pkgs_db <- setdiff(pkgs_db, removed_pkgs)
 
 pkgs <- setdiff(cran_pkgs, pkgs_db)
 formatted_pkgs <- gsub(