context: Create CACHEDIR.TAG file in state dir

[Vanadiae]

This makes it possible for backup software to detect and ignore the whole folder.

Fixes #360

[hfiguiere]

this sounds like a go idea.

[lctrr]

Great idea !

[TingPing]

Seems fine, sorry it was missed so long. Just needs rebased.

[lctrr]

Thank you very much ! for the rebase do you think the author is still active on GitHub though ?
@Vanadiae

[TingPing]

No idea, i can find time to rebase it if needed.

[Vanadiae]

I tried rebasing on the main branch, however for some reason the CI fails either with (CI logs):

Submodule 'debugedit' (https://sourceware.org/git/debugedit.git) registered for path 'subprojects/debugedit'
  Submodule 'libglnx' (https://gitlab.gnome.org/GNOME/libglnx.git) registered for path 'subprojects/libglnx'
  Cloning into '/home/runner/work/flatpak-builder/flatpak-builder/subprojects/debugedit'...
  Cloning into '/home/runner/work/flatpak-builder/flatpak-builder/subprojects/libglnx'...
  Error: error: Server does not allow request for unadvertised object ae27211cbbfb63a0ad3c141cd1310d7f583ec40e
  Error: fatal: Fetched in submodule path 'subprojects/debugedit', but it did not contain ae27211cbbfb63a0ad3c141cd1310d7f583ec40e. Direct fetching of that commit failed.
  Error: The process '/usr/bin/git' failed with exit code 128

or if I try scrapping my repo then clone again from upstream main branch, apply the patch and update the submodules (as this branch may have gotten too old by this point for some reason?) it now fails with (CI logs):

 echo '#include "config.h"' > libglnx-config.h
make  all-recursive
make[1]: Entering directory '/home/runner/work/flatpak-builder/flatpak-builder'
Making all in data
make[2]: Entering directory '/home/runner/work/flatpak-builder/flatpak-builder/data'
make[2]: Nothing to be done for 'all'.
make[2]: Leaving directory '/home/runner/work/flatpak-builder/flatpak-builder/data'
Making all in .
make[2]: Entering directory '/home/runner/work/flatpak-builder/flatpak-builder'
make[2]: *** No rule to make target 'subprojects/debugedit/tools/md5.c', needed by 'subprojects/debugedit/tools/flatpak_builder_debugedit-md5.o'.  Stop.
make[2]: *** Waiting for unfinished jobs....
make[2]: Leaving directory '/home/runner/work/flatpak-builder/flatpak-builder'
make[1]: *** [Makefile:2510: all-recursive] Error 1
make[1]: Leaving directory '/home/runner/work/flatpak-builder/flatpak-builder'
make: *** [Makefile:1172: all] Error 2

Interestingly it seems like the other two CI build jobs that use meson instead of the autoconf build work just fine as the meson job installs the debugedit directly with apt-get, contrary to the autotools build. I'm not familiar with these CI stuff however