[CI] Fix usage of `actions/cache` for conan packages

[feltech]

What

We occasionally see conan recipes being reinstalled/rebuilt.

Until now we've assumed that it's because a newer recipe revision was posted upstream. However, conan ignores the remote if the package is already installed (unless -u/--update is given). So a reinstall must mean the cache has disappeared.

We should understand why the cache disappears, and ideally prevent it.

Why

This would mitigate issues we have occasionally, whereby conan reinstalls a package whose recipe has updated such that the install fails, and so we're blocked on CI. This problem also spawned #551.

We should still update our conan packages, but on our schedule, not because we're forced to due to mysterious cache eviction logic.

For info: this was discovered whilst investigating #602, which led to the discovery that the cache is immutable, so any changes are never persisted. This may be relevant for any fix.