feat: Add UV_LINK_MODE=copy

[carlcsaposs-canonical]

Companion to canonical/craft-providers#900

When the cache dir is mounted from the LXD host, hardlinking from the global cache (uv's default behavior) is not possible

e.g.

:: warning: Failed to hardlink files; falling back to full copy. This may lead to degraded performance.
::          If the cache and target directories are on different filesystems, hardlinking may not be supported.
::          If this is intentional, set `export UV_LINK_MODE=copy` or use `--link-mode=copy` to suppress this warning.

[bepri]

Thanks! Could you make a changelog entry for this and update the reference docs for the plugin with a sentence or two explaining why it's set by us?

[carlcsaposs-canonical]

Thanks! Could you make a changelog entry for this and update the reference docs for the plugin with a sentence or two explaining why it's set by us?

Added in 4f5e86c—feel free to commit directly to this branch if you'd like to make any changes

[carlcsaposs-canonical]

(rebased on main)

[bepri]

Thanks - just minor nitpicks on the docs entries now.

[mr-cal]

LGTM but would like to get @lengau's review, since this is a charmcraft-motivated feature.

[lengau]

Setting this to copy by default will slow down the default case where the cache isn't mounted from the outer machine. While I think it's a good idea to document this here and provide a motivation for using copy, I don't think we should change the default value.

[carlcsaposs-canonical]

Once canonical/craft-providers#900 is merged, would that not become the default (similar to how pip's cache is mounted from the host by default)?

[lengau]

Let's leave the default value unset still.