Add option to restore cache only (for PRs)

[ForsakenHarmony]

Your feature request related to a problem? Please describe.

GitHub has a 10 GB limit for actions caches, this action alone is using 130 MB for us, it would be nice to not also get another 130 MB for each opened PR.

Describe the solution you'd like.

Add a save-cache option to be able to still restore cache (e.g. from main), but not save a new cache entry

Additional context.

actions/toolkit#1308
Swatinem/rust-cache#95 (has a save-if option https://github.com/Swatinem/rust-cache#example-usage)

[bombsimon]

Is the option skip-cache not working for you? From the docs:

# Optional: if set to true then the all caching functionality will be complete disabled,
#           takes precedence over all other caching options.
# skip-cache: true

[ForsakenHarmony]

@bombsimon I want to still restore the cache, but not save it

[krhubert]

This option would be great in order to reduce total cache size (on GH it's limited to 10GB).

Golang cache can take between 300-500 MB easily and because of that repo can quickly hit cache storage limit. One of possible solution to this problem is to save cache only on the main branch and use read-only mode for non main branches.