[FS#374] Github release tarballs should use content-disposition to set correct file name

[LEDE Bugs]

The following task has a new comment added:

FS#374 - Github release tarballs should use content-disposition to set correct file name
User who did this - Felix Fietkau (nbd)

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I think rather than using github release tarballs, we should just reference the git tree and rely on build system generated reproducible .xz tarballs.
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=374#comment1403