hostname is not handled correctly in /etc/hosts during install

[rolkinn]

Distribution (run cat /etc/os-release):
NAME="Pop!_OS"
VERSION="21.04"
ID=pop
ID_LIKE="ubuntu debian"
PRETTY_NAME="Pop!_OS 21.04"
VERSION_ID="21.04"
HOME_URL="https://pop.system76.com"
SUPPORT_URL="https://support.system76.com"
BUG_REPORT_URL="https://github.com/pop-os/pop/issues"
PRIVACY_POLICY_URL="https://system76.com/privacy"
VERSION_CODENAME=hirsute
UBUNTU_CODENAME=hirsute
LOGO=distributor-logo-pop-os

Related Application and/or Package Version (run apt policy $PACKAGE NAME):
hostname
/etc/hosts

Issue/Bug Description:
When choosing a hostname that is not "pop-os", /etc/hosts is not updated and continues to use the default "pop-os" hostname.

Output of mismatch after fresh install (or changing hostname via settings ui):
$ hostname

myhostname

$ cat /etc/hosts

127.0.0.1 localhost
::1 localhost
127.0.1.1 pop-os.localdomain pop-os

should be
$ cat /etc/hosts

127.0.0.1 localhost
::1 localhost
127.0.1.1 myhostname.localdomain myhostname

Impact:
This causes delays when resolving hosts.
For example, sudo will hang for an extended period of time if there is a mismatch in hostname and /etc/hosts.

Steps to reproduce (if you know):

1. Install Pop!_OS and choose any hostname that is not "pop-os". OR change hostname through System Settings.
2. Connect to internet. Optional: connect to internet and a VPN.
3. Run sudo. (this should hang for some amount of time before returning).

Expected behavior:
When hostname is updated, /etc/hosts should also be updated to reflect change (see above /etc/hosts output suggestion).

Other Notes:
The sudo command will sometimes not hang, however, it would always hang when also connected to a VPN.

[jacobgkau]

Possibly related to pop-os/systemd#5, if the hostname is set using hostnamectl or similar.