Replace crypt with passlib md5_crypt in scheduler

[kasparovabi]

#304 moved Server/bkr/server/model/identity.py from crypt.crypt to passlib.hash.md5_crypt. Server/bkr/server/model/scheduler.py still has import crypt at the top and calls crypt.crypt(group_pw, "$1$%s$" % salt) in Recipe.crypt_root_password. Python 3.13 removed the crypt module (PEP 594, What's New in 3.13), so on 3.13 the module import fails before anything in bkr.server.model loads:

$ docker run --rm --network none python:3.13-slim python -c "import crypt"
ModuleNotFoundError: No module named 'crypt'

This PR applies the same change as #304 to that one call site. passlib is already a requirement through identity.py (python3-passlib in beaker.spec and in the test-py3 container), so nothing new gets pulled in.

To check the hashes match I ran, on python:3.11-slim with passlib installed, 500 random password and 8-character salt pairs through both crypt.crypt(pw, "$1$<salt>$") and md5_crypt.using(salt=salt).hash(pw). All 500 were identical. The same call on python:3.13-slim gives the same string for a fixed input ($1$abcdefgh$1y4wFdC1gWzFZU0eph2bJ0 for beaker).

I left the import crypt lines in IntegrationTests alone (test_kickstart.py and three selenium tests) to keep this to the server code #304 touched. I can send those separately if you want them. I have not run the Beaker test suite against this branch.

I read this diff line by line before opening it, and I am happy to adjust or drop it. What brought me here was a checker I maintain that looks for imports of modules Python has removed, so I am saying that up front rather than leaving you to wonder. If you would rather not get this kind of contribution, tell me and I will not send another.