Add internal post-install step for Brioche installation and updates

[kylewlacy]

This PR introduces a new internal subcommand: brioche self-post-install. Currently, when called, it just prints a simple message:

Brioche v{CURRENT_VERSION} is now installed

This PR also updates the self-update subcommand to call brioche self-post-install on the newly-installed version. Since this command just prints a message, this doesn't do much today, but it does at least validate that the new version is working as expected (or prints a warning with some tips if the command fails... which could be a sign that something is very broken).

Of course, this doesn't provide much value today. But my hope is that the new self-update process (part of #281) can last well into the future, so this gives us a place to hook into for things like migrations or one-off cleanup tasks when going from older versions to newer versions. The benefit of doing this now is that future versions can always assume that the self-post-install command is called during the update process.

When calling self-post-install, the self-updater sets 2 extra env vars:

• BRIOCHE_SELF_POST_INSTALL_SOURCE=self-update (so the post-install step can tell apart e.g. a new install vs. an update)
• BRIOCHE_SELF_POST_INSTALL_PREVIOUS_VERSION (set to the version doing the self-update, so the post-install step can do different things depending on what version we're coming from)