snap/debuginfod: use user's $HOME/.cache/debuginfod_client as cache path

[xmkg]

Currently, the bundled debuginfod in hotkdump snap stores & uses the /home/<user>/snap/hotkdump/<revision>/.cache/debuginfod_client cache path. Ideally, the bundled debuginfod should also be able to access the user's .cache/debuginfod_client path to avoid duplicates, but that requires hotkdump snap to use personal-files interface, which needs to be approved by the snap store reviewers. Thus, we should decide if that's something worth digging into further.

PS: there's a workaround exists, which is to bind mount the path to the user's debuginfod cache (e.g. /home/user/.cache/debuginfod_client) to a path that hotkdump snap can access, e.g. /home/<user>/debuginfod-cache, and then set the DEBUGINFOD_CACHE_PATH environment variable before running the snap:

mkdir /home/user/debuginfod-cache
sudo mount --bind /home/user/.cache/debuginfod_client/ /home/user/debuginfod-cache
DEBUGINFOD_CACHE_PATH=/home/user/debuginfod-cache hotkdump --interactive -d /workspace/case-data/example-crash-dumps/lathiat/kdump-demos/focal/x86_64/5.15.0-67-generic/dump.202303240831