chore: Build flatpak package

[twpayne]

Refs #1800.

This PR attempts to build a Flatpak package of chezmoi.

As far as I can tell, Flatpak in its current form is incompatible with chezmoi:

• Flatpak's sandbox is too restrictive. Even with full filesystem access, chezmoi cannot execute other processes, which breaks password manager integration, encryption, and several template functions.
• Flatpak mangles XDG directory specification directories like $XDG_DATA_HOME, making Flatpak-installed chezmoi incompatible with conventionally-installed chezmoi.

There are likely other problems.

This is a draft PR for discussion.

[twpayne]

The CI failure seems to be because the available version of flatpak-builder on Ubuntu 20.04 (used for release building because its old version of glibc means the compiled binaries run on a wide range of systems):

Flatpak 1.6.5
flatpak-builder 1.0.10
...
Initializing build dir
Committing stage init to cache
Starting build of io.chezmoi.chezmoi
========================================================================
Building module chezmoi in /home/runner/work/chezmoi/chezmoi/.flatpak-builder/build/chezmoi-1
========================================================================
Running: install -Dm755 chezmoi /app/bin/chezmoi
Committing stage build-chezmoi to cache
Cleaning up
Committing stage cleanup to cache
Finishing app
error: Unknown filesystem location host-etc, valid locations are: host, home, xdg-*[/…], ~/dir, /dir
Error: Child process exited with code 1

[twpayne]

Following flatpak/flatpak#5999 (comment), it seems that chezmoi would need to bundle all applications it needs (including git, gpg, age, nvim, emacs, code, all password manager CLIs, etc.) which is neither desirable nor feasible, so I'll close this as "won't fix" for now.