fix: correct detection for Vite build vs serve mode

[draotth]

• Quick Checklist

• I have read the contributing guidelines
• I have written new tests, as applicable (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• I have added a changeset, if applicable

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Fix a bug where vite build -w produces /@imagetools/* URLs, when it should be producing image outputs.

• What is the new behavior (if this is a feature change)?

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this
  PR?)

• Other information:

[changeset-bot]

🦋 Changeset detected

Latest commit: 12930b4

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

This PR includes changesets to release 1 package

Name	Type
vite-imagetools	Patch

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[codecov]

Codecov Report

Merging #595 (12930b4) into main (76ccb91) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #595   +/-   ##
=======================================
  Coverage   93.93%   93.93%           
=======================================
  Files          32       32           
  Lines        1104     1104           
  Branches      185      185           
=======================================
  Hits         1037     1037           
  Misses         67       67           

Flag	Coverage Δ
imagetools-core	96.66% <ø> (ø)
vite-imagetools	79.42% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
packages/vite/src/index.ts	79.06% <100.00%> (ø)