Rollup merge of #135588 - ferrocene:add-license-json-to-source-tarball, r=Kobzol

Add license-metadata.json to rustc-src tarball.

Adds a license-metadata.json to the source tarball.

This file was reported as missing as a comment on #133461, and it prevents you building the compiler from the source tarball (unless you re-generate it yourself, which is non-obvious and requires `reuse` to be installed).

r? Kobzol