Make TLJH_BOOTSTRAP_DEV take effect for reals

bootstrap/bootstrap.py

@@ -87,14 +87,14 @@ def pip_install(prefix, packages, editable=False):
     Set editable=True to add '--editable' to the pip install commandline.
     Very useful when doing active development
     """
-    flags = '--no-cache-dir --upgrade'
+    flags = ['--no-cache-dir', '--upgrade']
     if editable:
-        flags += '--editable'
+        flags.append('--editable')
     subprocess.check_output([
         os.path.join(prefix, 'bin', 'python3'),
         '-m', 'pip',
-        'install', '--no-cache-dir',
-    ] + packages)
+        'install',
+    ] + flags + packages)
 
 
 def main():