checkout seems to be failing for ubuntu 20 recently:
```
Deleting the contents of '/__w/wezterm/wezterm'
Initializing the repository
/usr/bin/git init /__w/wezterm/wezterm
Initialized empty Git repository in /__w/wezterm/wezterm/.git/
/usr/bin/git remote add origin https://github.com/wez/wezterm
Error: fatal: unsafe repository ('/__w/wezterm/wezterm' is owned by someone else)
To add an exception for this directory, call:
git config --global --add safe.directory /__w/wezterm/wezterm
Error: The process '/usr/bin/git' failed with exit code 128
```
Try reverting this as a workaround.
`gh` is pre-installed in native runners only; when we build in a
container, we need to install it for ourselves.
This commit drops support for building on centos7 as it is a PITA
to get this working there.
I lost a few hours over the weekend because the GH release uploads
are flakey and the action I was using doesn't internally perform
retries. I had to manually delete the failed uploads from the release
and then re-trigger the builds across several platforms, several times
for both of the releases I pushed this weekend :-/
This commit speculatively switches over to using the GH cli in the
hopes that the error reporting is better, and also because it is
simpler to externally drive a retry loop.
Let's see how this goes.
