deploy: Find kernel/initramfs consistently from filesystem

deploy: Find kernel/initramfs consistently from filesystem

I'm porting the deployment code to be fd-relative, but part of the
logic was using `GFile` to talk to `OstreeRepoFile` to determine the
"bootcsum" (boot config checksum) before checking out the file tree.

We can avoid having both code paths by checking out the tree first,
then looking at it on the filesystem.