systemd/launchd: work around when promnesia binary isn't available in…

… PATH, use sys.executable

should resolve #433

src/promnesia/misc/install_server.py

@@ -3,7 +3,6 @@
 import argparse
 import os
 import platform
-import shutil
 import sys
 import time
 from pathlib import Path
@@ -114,14 +113,16 @@ def install(args: argparse.Namespace) -> None:
  print(f"Writing launch script to {out}", file=sys.stderr)
 
  # ugh. we want to know whether we're invoked 'properly' as an executable or ad-hoc via scripts/promnesia
+ extra_exe: list[str] = []
  if os.environ.get('DIRTY_RUN') is not None:
  launcher = str(root() / 'scripts/promnesia')
  else:
- exe = shutil.which('promnesia'); assert exe is not None
- launcher = exe # older systemd wants absolute paths..
+ launcher = sys.executable
+ extra_exe = ['-m', 'promnesia']
 
  db = args.db
  largs = [
+ *extra_exe,
  'serve',
  *([] if db is None else ['--db', str(db)]),
  '--timezone', args.timezone,