nix(loadtest): generate targets at nix build time

Instead of generating them at loadtest runtime, we generate these
dynamic targets once via Nix. Because we're using libfaketime, we're not
bound to doing it at the same time as running the tests.

This allows us to run each loadtest right after each other, with minimal
other stuff going on inbetween. It's also a tiny bit more reproducible,
because the loadtests against both branches run with exactly the same
set of targets, where they did not before.

nix/tools/loadtest.nix

@@ -3,7 +3,9 @@
 , git
 , jq
 , libfaketime
+, python3
 , python3Packages
+, runCommand
 , vegeta
 , withTools
 , writers
@@ -65,15 +67,13 @@ let
 
         case "$_arg_kind" in
           jwt-cache)
-            export PGRST_JWT_SECRET="@test/load/gen_jwks.json"
-
-            ${libfaketime}/bin/faketime '2000-01-01 00:00:00' ${genTargets} test/load
+            export PGRST_JWT_SECRET="@${generatedTargets}/gen_jwks.json"
 
             # shellcheck disable=SC2145
             ${withTools.withPg} -f test/load/fixtures.sql \
             ${withTools.withPgrst} --faketime '2000-01-01 00:00:00' -m "$_arg_monitor" \
             sh -c "cd test/load && \
-            ${runner} -targets gen_targets.http -output \"$abs_output\" \"''${_arg_leftovers[@]}\""
+            ${runner} -targets ${generatedTargets}/gen_targets.http -output \"$abs_output\" \"''${_arg_leftovers[@]}\""
             ;;
 
           # here we sleep purposefully to check how much memory does the schema cache consume in the final report
@@ -291,13 +291,15 @@ let
           | ${mergeMonitorResults}
       '';
 
-  genTargets =
-    writers.writePython3 "postgrest-gen-loadtest-targets"
+  generatedTargets =
+    runCommand "postgrest-loadtest-targets"
       {
-        libraries = [ python3Packages.jwcrypto ];
-        doCheck = false; # postgrest-style conflicts with this
+        nativeBuildInputs = [ (python3.withPackages (pyps: [ pyps.jwcrypto ])) ];
       }
-      (builtins.readFile ./generate_targets.py);
+      ''
+        mkdir -p "$out"
+        ${libfaketime}/bin/faketime '2000-01-01 00:00:00' python3 ${./generate_targets.py} "$out"
+      '';
 
   mergeMonitorResults =
     writers.writePython3 "postgrest-merge-monitor-results"