If the systemd version on the host is too old and there's no local
build directory, use the default tools tree which will build an
image containing all the tooling required to build systemd and use
that to build the other presets.
Although, this is CI, we can still do better. It also ensures that any
env var changes make it into the script, as things like PATH would not
survive a `sudo -E`.
Configuration now takes priority over CLI options so we have to
configure the defaults for settings that we want to allow overriding
from the CLI. We also explicitly set some other settings so that they
can't be overridden from the CLI anymore. For example the base and
initrd image should never be made bootable so we set Bootable=no
explicitly for both.
The mkosi Arch CI doesn't work as the keyring package is out
of date and cannot be built due to various build toolchain
issues. Disable the job as it always fails and confuses
submitters.
Now that we use meson feature options for our dependencies, we can just
rely on '--auto-features=disabled' to do the same. One benefit of this
is that specific features can still be force-enabled by overriding it
with the appropriate '-Dfeature=enabled' flag.
The two remaining uses for skip-deps can simply rely on their default
logic that sets the value to 'no' when the dependency is disabled.
By using meson features we can replace the handcrafted dependency
auto-detection by just passing the value from get_option directly to the
required arg for dependency, find_library etc.
'auto' features make the dependency optional, 'enabled' requires it
while 'disabled' features will skip detection entirely.
Any skipped or not found dependency will just be a no-op when passed to
build steps and therefore we can also skip the creation of empty vars.
The use of skip_deps for these is dropped here as meson provides a way
to disable all optional features in one go by passing
'-Dauto_features=disabled'.
This update introduces the explicit Dependencies= setting, instead
of relying on implicit dependencies via alphanumerical ordering.
We also take the opportunity to rename the "final" preset to the
"system" preset, which seems like a better name.
Exclude all `.in` files because they may contain unsupported syntax, and
they have to be preprocessed first. For example:
```sh
Error: SHELLCHECK_WARNING:
./src/rpm/systemd-update-helper.in:130:37: warning[SC1083]: This { is literal. Check expression (missing ;/\n?) or quote it.
```
Related to: https://github.com/systemd/systemd/pull/28521
