feat(daemon): Add support for `PEBBLE_COPY_ONCE`.

[hpidcock]

$PEBBLE_COPY_ONCE environment variable allows pebble run to copy layers and state from another directory when seeding a new $PEBBLE directory.

The rationale for this change is to allow OCI images to specify existing layers in the default $PEBBLE location. Then at runtime, be able to run pebble with a different $PEBBLE directory. This allows pebble to be run as a non-root user or run on a read-only root filesystem.

Issue #351 outlines that pebble run has no unit or integration tests. A test should be performed manually for now.

Manual QA is as follows:

• Create a directory, with a layers subdirectory and valid testing layers (e.g. ~/dir/with-existing-state)
• Create another empty directory. (e.g. ~/dir/with-no-state)
• PEBBLE_COPY_ONCE=~/dir/with-existing-state PEBBLE=~/dir/with-no-state pebble run
• Check all the layers are loaded.
• Check all the layers are copied.
• Modify a layer in ~/dir/with-no-state/layers.
• Restart pebble run and check that it doesn't crash.
• Check that the modifications are preserved after the restart (PEBBLE_COPY_ONCE is only for an empty directory).
• Check PEBBLE_COPY_ONCE can be set to a non-existing directory with pebble run passed --create-dirs

JU090 JUJU-5435

[benhoyt]

One other thought: we should document this in README.md somewhere. Or maybe in the help text?