fix: populate release cache on 'show' for non-installed packages

[mvanhorn]

Closes #1842

Problem

deb-get show <pkg> prints empty Published: and Download: lines when the package is not installed and uses get_github_releases, get_gitlab_releases, or get_website:

jet@debian13:~$ deb-get show gb-studio
  [*] WARNING! Cached file /var/cache/deb-get/gb-studio.json_extract is empty or missing.
GB Studio
  Package:      gb-studio
  Repository:   01-main
  Updater:      deb-get
  Installed:    No
  Published:
  Architecture: amd64 arm64
  Download:
  Website:      https://www.gbstudio.dev/
  Summary:      ...

Per-package scripts like 01-main/packages/gb-studio derive URL and VERSION_PUBLISHED by reading ${CACHE_FILE}, but since #1419 the cache is only populated for install, fix-installed, and update (the latter only when the package is already installed). show was not in that allowlist, so for non-installed packages the cache file never existed, the grep in the package definition returned nothing, and the two lines came out blank.

Fix

Add show to the action allowlist in get_github_releases, get_gitlab_releases, and get_website. The existing "no cache for apps we never install" guarantee (no fetching during list, prettylist, csvlist, search, etc.) is preserved.

Testing

• bash -n deb-get passes
• The three helpers are the only places where cache population is gated by ${ACTION}; the same pattern is applied consistently across all three

Scope notes

• No change for list, prettylist, csvlist, search, remove, purge — cache is still not fetched during those actions
• No change for packages that use direct sources — they do not depend on the cache file
• Per-hour cache reuse (DEBGET_CACHE_RTN) still applies, so repeated show calls won't hammer GitHub / GitLab APIs

[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Auto-approved: Trivial fix adding 'show' to an allowlist for metadata cache population, resolving a UI issue with empty fields for non-installed packages.}

[mvanhorn]

Thanks for the merge, @philclifford. Caching on show makes the UX a lot snappier.