fix(transport): disable avif when png

[ykzts]

Kagami/go-avif does not yet support the alpha channel.

see https://github.com/Kagami/go-avif/blob/v0.1.0/avif.go#L102-L103

[vercel]

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[codecov]

Codecov Report

Merging #462 (522f174) into main (fc79165) will not change coverage.
The diff coverage is 50.00%.

@@           Coverage Diff           @@
##             main     #462   +/-   ##
=======================================
  Coverage   75.37%   75.37%           
=======================================
  Files           6        6           
  Lines         134      134           
=======================================
  Hits          101      101           
  Misses         23       23           
  Partials       10       10           

Impacted Files	Coverage Δ
transport.go	77.27% <50.00%> (ø)

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