Run & fix `staticcheck`

.github/workflows/main.yml

api/_routers/01-install_metadata.go

api/_routers/03-host_detection.go

api/_routers/98-use-rcontext.go

api/custom/exports.go

api/custom/usage.go

api/unstable/info.go

archival/entity_export.go

archival/v2archive/writer.go

cmd/export_synapse_for_import/main.go

cmd/import_synapse/main.go

cmd/media_repo/main.go

cmd/plugin_antispam_ocr/main.go

cmd/s3_consistency_check/main.go

common/config/access.go

common/config/watch.go

common/context.go

common/rcontext/request_context.go

datastores/pick.go

datastores/upload.go