Add option to read environment from cache but not set new cache

[jonashaag]

Use case: In PRs, we might want to not create any new cache entries so as to not burn through the cache limits so quickly.

Similar to https://github.com/hendrikmuhs/ccache-action/blob/572582a14cd1c7a54f51762befec013fff6ba6e5/action.yml#L20-L22