--import-cache type=local should not error out when the cache doesn't exist

[AkihiroSuda]

$ buildctl-daemonless.sh b \
  --frontend dockerfile.v0 --local context=. --local dockerfile=. \
  --export-cache type=local,dest=/tmp/foo \
  --import-cache type=local,src=/tmp/foo
[+] Building 0.0s (0/0)                                                                                                                                                                                
error: could not read /tmp/foo/index.json: open /tmp/foo/index.json: no such file or directory

[nvpandeti]

Hi I'm currently taking a Virtualization course at UT Austin that requires us to contribute to an open source project. If possible I'd like to take a shot at this issue.

[AkihiroSuda]

thanks, please