apk: format install_if fields without brackets

[officialasishkumar]

Summary

• format APKINDEX and installed DB install_if fields as space-separated values instead of Go slice strings
• update generated image fixtures and publish digest expectations for the byte-level metadata change

Fixes #2059

Test plan

• go test ./pkg/apk/...
• go test ./...
• git diff --check
• go mod tidy && git diff --exit-code -- go.mod go.sum