Preserve flatpak cache from mkosi chroot across rebuilds

[vjr]

Fixes #895 by speeding up incremental builds.

[github-actions]

Installer builds for 6412df6

Do not use these installers for production systems. They are intended for testing purposes only. They will update to the latest PR build, which will break your install eventually.

Arch	ISO	Checksum
amd64	elementaryos-9.0-proposed-x86-64.20260928120308.iso	sha256
arm64	elementaryos-9.0-proposed-arm64.20260928120235.iso	sha256

[vjr]

Looks like elementary/elementary-build-containers#3 is no longer needed so that can be discarded once this PR is either merged or rejected.

[github-actions]

Installer builds for e9e9d98

Do not use these installers for production systems. They are intended for testing purposes only. They will update to the latest PR build, which will break your install eventually.

Arch	ISO	Checksum
amd64	elementaryos-9.0-proposed-x86-64.20260928124227.iso	sha256
arm64	elementaryos-9.0-proposed-arm64.20260928124012.iso	sha256

[vjr]

Initial clean builds take about 15 minutes for me.

Prior to this PR subsequent builds dropped from 15 down to about 8 minutes mainly due to APT caching but lack of flatpak caching.

With the changes in this PR subsequent build times dropped further down to under 3 minutes.

[vjr]

Retargeted this to merge into #873 instead of main.

[github-actions]

Installer builds for 83385a5

Do not use these installers for production systems. They are intended for testing purposes only. They will update to the latest PR build, which will break your install eventually.

Arch	ISO	Checksum
amd64	elementaryos-9.0-proposed-x86-64.20260929202829.iso	sha256
arm64	elementaryos-9.0-proposed-arm64.20260929202447.iso	sha256