Explicitly set builder for Fly (#1863)

* Try Python 3.9 in fly build to reduce memory usage

* Revert test data insertion and instead increase fly swap

* Use explicit builder for Fly

.github/fly.toml

@@ -1,11 +1,12 @@
-# fly.toml file generated for broken-field-1976 on 2022-11-21T21:37:55Z
-
 app = "optimade"
 kill_signal = "SIGINT"
 kill_timeout = 5
 swap_size_mb = 1024
 processes = []
 
+[build]
+  builder = "paketobuildpacks/builder:base"
+
 [env]
 
 [experimental]
@@ -37,9 +38,3 @@ processes = []
     interval = "15s"
     restart_limit = 0
     timeout = "2s"
-
-[build]
-  builtin = "python"
-
-[build.settings]
-  pythonbase = "3.9-slim-buster"