Track nixos-unstable channel as nixpkgs

[montchr]

22.11 is almost out. 21.11 has been deprecated.

Since nixpkgs was so out of date, the deprecation warnings alluded to in #126 were not clear here.

[montchr]

From https://github.com/ryantm/agenix/actions/runs/3256568521/jobs/5347221800:

building '/nix/store/dcr5dfhm39iz7dppppxb0q0d6ia26nld-nixos-test-driver-agenix-integration.drv'...
error: builder for '/nix/store/dcr5dfhm39iz7dppppxb0q0d6ia26nld-nixos-test-driver-agenix-integration.drv' failed with exit code 1;
       last 10 log lines:
       >     60    assert "user1" in system1.succeed("cat /tmp/1")
       > testScriptWithTypes:52: error: Argument 1 to "wait_until_tty_matches" of
       > "Machine" has incompatible type "int"; expected "str"
       >     system1.wait_until_tty_matches(2, "login: ")
       >                                    ^
       > testScriptWithTypes:54: error: Argument 1 to "wait_until_tty_matches" of
       > "Machine" has incompatible type "int"; expected "str"
       >     system1.wait_until_tty_matches(2, "login: user1")
       >                                    ^
       > Found 2 errors in 1 file (checked 1 source file)
       For full logs, run 'nix log /nix/store/dcr5dfhm39iz7dppppxb0q0d6ia26nld-nixos-test-driver-agenix-integration.drv'.
error: 1 dependencies of derivation '/nix/store/llh018jh2y66b8qkagwdv5jfhz18rh2w-vm-test-run-agenix-integration.drv' failed to build

[ryantm]

I'm okay with this if we switch back to 22.11 before we make a release of agenix.