Fix assets not getting optimized when outDir is outside the CWD

[MichailiK]

Changes

• Resolves astro:assets breaks when outDir is set to a folder that's outside the project's folder #8668
• Fixes a build failure when using astro:assets and setting outDir to a folder outside of the project. For example outDir: '../outside-the-project'
• It's caused by some assets being constrained in the project folder/cwd, by using getOutDirWithinCwd
• astro:assets does not use getOutDirWithinCwd, so it ends up looking for the unoptimised assets at the wrong place

I wrote a slightly deeper explanation in #8668 (comment).

Testing

The minimal reproducible example in the issue above (https://github.com/MichailiK/astro-images-outdir-bug) was used to test this change. I don't think this would cause regressions. I believe this change just ensures that Astro is trying to obtain the unoptimised asset from the correct folder.

Docs

Since this is a bug, caused by an inconsistency between where astro:assets is looking for the assets & where they are actually located, I don't believe this needs docs changes.

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[matthewp]

Thanks!