AADLoginForLinux not working with Ubuntu 20.04 image?

Hello!

I've recently been testing some virtual machines using Ubuntu 20.04 LTS, and it seems like the `AADLoginForLinux` extension doesn't appear to work correctly with this version of Ubuntu.

I first tried creating a VM with (substitute values as needed):
```
az vm create -g resourcegroup -n mynewvm \
--image "Canonical:0001-com-ubuntu-server-focal:20_04-lts-gen2:latest" \
--admin-username admin --admin-password adminpassword --subnet \
"/subscriptions/subscriptionid/resourceGroups/resourcegroup/providers/Microsoft.Network/virtualNetworks/vnet/subnets/subnet"
```
And then installing the `AADLoginForLinux` extension thusly:
```
az vm extension set -n AADLoginForLinux --publisher Microsoft.Azure.ActiveDirectory.LinuxSSH \
--resource-group resourcegroup --vm-name mynewvm
```
And after enabling JIT Access for the new VM in the Azure Portal, tried ssh'ing into the VM with my Azure AD credentials:
```
ssh 9numbernine9@myorg.org@12.34.56.78 # Or whatever the IP address of this new VM happens to be
```
Instead of receiving the usual `use a web browser to open the page https://microsoft.com/devicelogin` prompt, I'm simply prompted for a password for my account. Using my password for my AD account, unsurprisingly, doesn't work.

Using the `admin` account that I created, however, if I log into the VM and `tail -f /var/log/auth.log` while trying to log in with my AD account, I see the following log entries:
```
Jan 12 19:26:21 mynewvm sshd[3128]: nss_aad: Managed System Identity is not enabled for this VM. Returning a user that may or may not exist.
Jan 12 19:26:21 mynewvm aad_certhandler[3130]: Version: 1.0.014460002; user: 9numbernine9@myorg.org
Jan 12 19:26:21 mynewvm aad_certhandler[3130]: The public key is of type ssh-ed25519, not a certificate.
Jan 12 19:26:21 mynewvm sshd[3128]: AuthorizedKeysCommand /usr/sbin/aad_certhandler 9numbernine9@myorg.org AAAAC3NzaC1lZDI1NTE5AAAAIJVu1e2oZYclyDdRVrSHy+M+OrByjAeijdfjxDrZ/Tkl failed, status 2
Jan 12 19:26:21 mynewvm sshd[3131]: pam_aad(sshd:auth): AadAuthenticate, Version: 1.0.014460002; CorrelationId: f241c6d6-9429-47f7-a894-52ce58449367
Jan 12 19:26:21 mynewvm sshd[3131]: pam_aad(sshd:auth): Cannot find the bearer challenge. ResponseHeaders: HTTP/2 401 #015#012cache-control: no-cache#015#012pragma: no-cache#015#012content-type: application/json; charset=utf-8#015#012expires: -1#015#012www-authenticate: Bearer authorization_uri="https://login.windows.net/7cc12d39-5552-4b22-80b6-70296cdd3312", error="invalid_token", error_description="The authentication failed because of missing 'Authorization' header."#015#012x-ms-failure-cause: gateway#015#012x-ms-request-id: 400076e0-9da6-4c0c-99e8-69c826bc76c9#015#012x-ms-correlation-request-id: 400076e0-9da6-4c0c-99e8-69c826bc76c9#015#012x-ms-routing-request-id: CANADACENTRAL:20210112T192621Z:400076e0-9da6-4c0c-99e8-69c826bc76c9#015#012strict-transport-security: max-age=31536000; includeSubDomains#015#012x-content-type-options: nosniff#015#012date: Tue, 12 Jan 2021 19:26:20 GMT#015#012content-length: 115#015#012#015
Jan 12 19:26:21 mynewvm sshd[3128]: error: PAM: Authentication service cannot retrieve authentication info for 9numbernine9@myorg.org from 111.111.111.111
Jan 12 19:26:22 mynewvm sshd[3128]: Connection closed by authenticating user 9numbernine9@myorg.org 111.111.111.111 port 62461 [preauth]
```
I'm not exactly sure what's going wrong here in this scenario, but it seems like `pam_aad` is perhaps having difficulty reaching AD or isn't configured correctly after installating the `AADLoginForLinux` extension? Note that the exact same set of steps works for a Ubuntu 18.04 VM (`Canonical:UbuntuServer:18_04-lts-gen2:18.04.202101081`) and `/var/log/auth.log` shows everything working correctly:
```
Jan 12 21:02:25 mynewvm_1804 sshd[3076]: nss_aad: Managed System Identity is not enabled for this VM. Returning a user that may or may not exist.
Jan 12 21:02:25 mynewvm_1804 aad_certhandler[3084]: Version: 1.0.014460002; user: 9numbernine9@myorg.org
Jan 12 21:02:25 mynewvm_1804 aad_certhandler[3084]: The public key is of type ssh-ed25519, not a certificate.
Jan 12 21:02:25 mynewvm_1804 sshd[3076]: AuthorizedKeysCommand /usr/sbin/aad_certhandler 9numbernine9@myorg.org AAAAC3NzaC1lZDI1NTE5AAAAIJVu1e2oZYclyDdRVrSHy+M+OrByjAeijdfjxDrZ/Tkl failed, status 2
Jan 12 21:02:25 mynewvm_1804 sshd[3085]: pam_aad(sshd:auth): AadAuthenticate, Version: 1.0.014460002; CorrelationId: 13b63ff9-4ede-4298-92b9-2d994dc2593a
Jan 12 21:02:39 mynewvm_1804 sshd[3085]: pam_aad(sshd:account): AadAuthorize, Version: 1.0.014460002; CorrelationId: 13b63ff9-4ede-4298-92b9-2d994dc2593a
Jan 12 21:02:39 mynewvm_1804 sshd[3085]: pam_aad(sshd:account): Login granted for 9numbernine9@myorg.org as an admin.
Jan 12 21:02:39 mynewvm_1804 sshd[3085]: nss_aad: Managed System Identity is not enabled for this VM. Returning a user that may or may not exist.
Jan 12 21:02:39 mynewvm_1804 sshd[3085]: pam_aad(sshd:account): First call for 9numbernine9@myorg.org. Provisioning user data.
Jan 12 21:02:39 mynewvm_1804 aaduseradd[3091]: new user: name=9numbernine9@myorg.org, UID=3544131, GID=3544131, home=/home/9numbernine9, shell=/bin/bash
Jan 12 21:02:39 mynewvm_1804 sshd[3085]: pam_aad(sshd:account): Adding 9numbernine9@myorg.org to aad_admins.
Jan 12 21:02:39 mynewvm_1804 gpasswd[3097]: user 9numbernine9@myorg.org added by root to group aad_admins
Jan 12 21:02:39 mynewvm_1804 sshd[3085]: pam_aad(sshd:account): Adding 9numbernine9@myorg.org to admin.
Jan 12 21:02:39 mynewvm_1804 gpasswd[3101]: user 9numbernine9@myorg.org added by root to group admin
Jan 12 21:02:39 mynewvm_1804 sshd[3076]: Accepted keyboard-interactive/pam for 9numbernine9@myorg.org from 111.111.111.111 port 22626 ssh2
Jan 12 21:02:39 mynewvm_1804 sshd[3076]: pam_unix(sshd:session): session opened for user 9numbernine9@myorg.org by (uid=0)
```

Any suggestions or insights into this issue would be greatly appreciated!

Start by reproducing the AADLoginForLinux installation on Ubuntu 20.04 with the provided az vm and SSH commands, then compare /var/log/auth.log with the working Ubuntu 18.04 case. Trace the pam_aad authentication flow around the missing bearer challenge. Done means Azure AD SSH login reaches the device-login prompt and completes successfully on Ubuntu 20.04.

Written by the indexing model from the issue text.