Finalize Ubuntu updates
add database stamp for core.sr.ht
master-shell: replace tabs with spaces in manifest
master-shell: handle case where no cmd is provided
Add "master-shell", supports submitting via SSH
Improve builds.sr.ht shell support

- Allow multiple connections
- Allow user to pass commands other than the shell
Update version number of libnss_dns in COPY

Fixes error building image on Ubuntu 20.04.

Also added comment noting that it is safe and recommended to update
that version number, and asking for a patch to be submitted.
Initial support for job group triggers
Initial support for job groups
worker: trim wc output before splitting it

Apparently OpenBSD indents the output of wc; see
crontab: drop ubuntu/latest
Ubuntu: switch build.yml to LTS

The latest symlink is going to be dropped in the foreseeable future
worker: omit tasks from email if they begin with _
Expand capabilities of the email trigger

Adds In-Reply-To and Cc options, and fixes multiple recipients
Make nix installation more robust all around

* Make curl follow redirects (this is what broke the builds)
* Separate the curl and sh steps
* Say explicitly that the daemon isn't needed
* No channels by default
* Use ping-store to check the installation
Add ubuntu/20.10 alias
Update contrib/crontab
ubuntu: sync with reality
alpine: remove 3.8, add more symlinks
Update .gitignore