fix: verify the metadata database certificate with sslmode=verify-full (#235)

Part of https://codefloe.com/rpkgs/bincraft/issues/225. Stacked on #234.

Every connection to the metadata database used `sslmode = 'require'`, which encrypts but does not authenticate the server, so anyone able to impersonate `r-binaries.devxy.io:15432` would receive `PGPASS`.

## Changes

- `.crow/build-all-versions.yaml`, `process-updates.yaml`, `prune-stale-flat-binaries.yaml`, `reindex.yaml`, `weekly-rebuild-reindex.yaml`: `metadata_db_sslmode = 'verify-full'` in every bincraft call.
- `local/*.R`: the same for bincraft calls; scripts that call `DBI::dbConnect()` directly also pass `sslrootcert = bincraft::postgres_sslrootcert("verify-full")`, because libpq looks for `~/.postgresql/root.crt` rather than the system trust store.
- `README.md`: note that the endpoint serves a Let's Encrypt certificate and how to verify it.
- air reformatted three `=` assignments to `<-` in `local/missing-cran-packages-db.R` and `local/packages-without-any-binary.R` while the hook ran on those files.

## Ordering: merge last

1. devxy/k8s-talos#417 must be live: today the server presents an in-cluster certificate from a private CA, which `verify-full` rejects.
2. https://codefloe.com/rpkgs/bincraft/pulls/226 must be released: `local/install-bincraft.R` installs the latest tag, and older releases neither pass a root certificate nor export `postgres_sslrootcert()`.
3. #234 must be merged.

Merging early fails every database connection in CI, and since #234 per-minor reindex failures fail the pipeline.

Reviewed-on: #235

local/backfill-abi-classification.R

@@ -47,7 +47,7 @@ db <- list(
   metadata_db_port = 15432L,
   metadata_db_user = "rpkgs",
   metadata_db_password = Sys.getenv("PGPASS"),
-  metadata_db_sslmode = "require"
+  metadata_db_sslmode = "verify-full"
 )
 
 read_index <- function(index_url) {