Put build history into the output directory

Put build history into the output directory

Running e.g. `test_initrd` and `test_initrd_luks` in parallel fails one of
them with "Image 'main' has not been built yet". The integration tests build
into a per-test `--output-directory`, but `vm()`/`boot()` did not pass it, so
those verbs recovered the build configuration from the *shared* global history
in `<configdir>/.mkosi-private/history/latest.json`. With concurrent builds
that file holds whatever the last build wrote, so a verb reads back another
build's config (e.g. the wrong `Format=`).

Tie the build history to the output directory: when an output directory is
given on the CLI, store and read the history under it instead of in the config
directory. Each build's history is then isolated, and a verb pointed at a
given `--output-directory` reads back exactly that build's configuration. In
the tests, pass `--output-directory` to `vm()` and `boot()` as well.

As a consequence, `mkosi vm` (and the other verbs that consume a previous
build) now require `-O`/`--output-directory` when the build used one. This is
a behaviour change, but unbreaks having more than one output dir.

Note: If a config file sets `OutputDirectory=`, the history continues to
be in the config dir, as before. The computation of the history
directory (necessarily) happens before parsing the config
files/includes. This *only* applies to the CLI option.

Rejected alternatives:
 * This cannot be worked around with `--history=no` in the tests': that
   only disables *writing* history, not *reading* it, so vm/boot still
   pick up a stale (in our setup, empty) `latest.json` and fall back to
   the wrong config.
 * A dedicated `--history-dir` option would just be redundant with
   `--output-dir`.