Fix external astro image usage

[crutchcorn]

Changes

This PR is meant as somewhat of a companion piece to #4146 in order to enable running @astrojs/image inside of astro.config files.

Specifically, this PR works around the issue of import.meta.env not being defined in non-Vite environments like astro.config.

Testing

This was added to my local environment, and, alongside #4146 was the final blocker I needed to bypass in order to create a local rehype-astro-image plugin.

No tests were added, since I was not sure if this required tests or even how to properly add tests that rely on non-Vite environments. Happy to add them with a bit of guidance 😄

Docs

This is a bug fix, thusly no docs are needed

[changeset-bot]

🦋 Changeset detected

Latest commit: 2a980dd

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@astrojs/image	Patch

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[matthewp]

Thanks! I'm assuming the Windows test failing is flakiness, will rerun.

[tony-sull]

@crutchcorn thanks for opening this PR! How exactly will getImage be used outside of Vite?

Currently the integration is depending on a custom vite plugin to parse metadata from local image files, I don't think getImage would be able to handle local images without a larger change to move away from the Vite dependency there

[crutchcorn]

@tony-sull So I actually wrote a Remark plugin that appears to work quite well for my needs outside of Vite:

https://github.com/unicorn-utterances/unicorn-utterances/blob/astro/astro.config.ts#L91-L158

Specifically, the only change needed was this line to work around non-Vite execution:

https://github.com/unicorn-utterances/unicorn-utterances/blob/astro/astro.config.ts#L114-L117