Describe the enhancement
On bigger repos, the maximum cache size of 10 GB is easily exceeded with PRs.

It would be nice if there was a global environment variable one could use to prevent saving cache in PRs.

I'm creating this issue here because if this was implemented at the library level, any action that uses this would automatically benefit from it without having to implement it themselves. I'm going around figuring out which actions are creating caches right now and creating issues for all of them.

Additional information
Swatinem/rust-cache#95