installer script: try fixing intermittent errors

assemble_installer.py

@@ -1,16 +1,28 @@
 import requests
 import subprocess
 import shutil 
+import sys
 
 response = requests.get('https://hydra.nixos.org/jobset/experimental-nix-installer/experimental-installer/evals', headers={'Accept': 'application/json'})
 
 hydra_eval = response.json()['evals'][0]
 
+installers = []
+
 for build_id in hydra_eval['builds']:
     response = requests.get(f"https://hydra.nixos.org/build/{build_id}", headers={'Accept': 'application/json'})
     build = response.json()
     installer_url = build['buildoutputs']['out']['path']
     system = build['system']
-    subprocess.call(f"nix-store -r {installer_url}", shell=True)
+    if build['finished'] == "1":
+        try:
+            subprocess.call(f"nix-store -r {installer_url}", shell=True)
+        except:
+            # retry once
+            subprocess.call(f"nix-store -r {installer_url}", shell=True)
+        installers.append((installer_url, system))
+    else:
+        sys.exit(0)
 
-    shutil.copy(f"{installer_url}/bin/nix-installer", f"nix-installer-{system}")
+for installer_url, system in installers:
+    shutil.copy(f"{installer_url}/bin/nix-installer", f"nix-installer-{system}")