Fix Net::SSH::HostKeyMismatch between bin/test runs

[mdkent]

Doing some local bin/test runs and I'm seeing

 DEBUG [0b6e3a84] 	About to lock...
  INFO [0b6e3a84] Finished in 0.003 seconds with exit status 0 (successful).
  INFO [e803fce6] Running /usr/bin/env mkdir -p .kamal on vm1
  INFO [83041641] Running /usr/bin/env mkdir -p .kamal on vm2
  ERROR (Net::SSH::HostKeyMismatch): Exception while executing on host vm1: fingerprint SHA256:OIsqP8zwE4/STZIlBWDcsOwya+FijPX2zY0KukLit38 does not match for "vm1,172.28.0.2"
/usr/local/bundle/gems/net-ssh-7.2.1/lib/net/ssh/verifiers/always.rb:53:in `process_cache_miss'
/usr/local/bundle/gems/net-ssh-7.2.1/lib/net/ssh/verifiers/always.rb:35:in `verify'
/usr/local/bundle/gems/net-ssh-7.2.1/lib/net/ssh/verifiers/accept_new.rb:17:in `verify'
<snip>
Finished in 83.636942s, 4.8424 runs/s, 13.0564 assertions/s.
405 runs, 1092 assertions, 0 failures, 4 errors, 0 skips

Looks like the issue is some known_hosts build up after dozens of bin/test runs:

mk-mbp3 3.2.0 ~/Work/basecamp/kamal/test/integration missing-base64-require * TEST_ID=64da9d9973c6e80d4dbf6b338e5905a3 docker compose exec deployer bash
<snip>
root@8ff8d63beea5:~# ls -al .ssh/
total 40
drwxr-xr-x 2 root root  4096 Nov 10 19:25 .
drwxr-xr-x 4 root root  4096 Nov 10 19:25 ..
-rw------- 1 root root  2610 Jan  8 20:04 id_rsa
-rw-r--r-- 1 root root   574 Jan  8 20:04 id_rsa.pub
-rw-r--r-- 1 root root 18449 Jan  8 20:01 known_hosts
root@8ff8d63beea5:~# cat .ssh/known_hosts
vm2,172.19.0.2 ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOEOtc2wsqJe/snRuCZRzppjMq1i8P12E27lSBGhRbK7
vm1,172.19.0.3 ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOEOtc2wsqJe/snRuCZRzppjMq1i8P12E27lSBGhRbK7
vm1,172.21.0.3 ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOEOtc2wsqJe/snRuCZRzppjMq1i8P12E27lSBGhRbK7
<snip>
vm2,172.29.0.4 ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOEOtc2wsqJe/snRuCZRzppjMq1i8P12E27lSBGhRbK7
vm1,172.29.0.3 ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOEOtc2wsqJe/snRuCZRzppjMq1i8P12E27lSBGhRbK7
vm2,172.30.0.4 ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOEOtc2wsqJe/snRuCZRzppjMq1i8P12E27lSBGhRbK7
<snip>
vm1,192.168.16.4 ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOEOtc2wsqJe/snRuCZRzppjMq1i8P12E27lSBGhRbK7
vm2,192.168.16.2 ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOEOtc2wsqJe/snRuCZRzppjMq1i8P12E27lSBGhRbK7
vm2,192.168.32.4 ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOEOtc2wsqJe/snRuCZRzppjMq1i8P12E27lSBGhRbK7
vm1,192.168.32.2 ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOEOtc2wsqJe/snRuCZRzppjMq1i8P12E27lSBGhRbK7
vm1,192.168.48.3 ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIOEOtc2wsqJe/snRuCZRzppjMq1i8P12E27lSBGhRbK7
<snip>

Added a cleanup.