This refactoring from #5766 accidentally broke the tag name calculation
for the `build-src-tarball.sh` script so this fixes it by copying over
the same logic from `build-tarballs.sh`.
* Add source tarballs to our releases
This commit adds a small script to create a source tarball as part of
the release process. This goes further than requested by #3808 by
vendoring all Rust dependencies as well to be more in line with
"download the source once then build somewhere without a network".
Vendoring the Rust dependencies makes the tarball pretty beefy (67M
compressed, 500M uncompressed). Unfortunately most of this size comes
from vendored crates such as v8, pqcrypto-kyber, winapi, capstone-sys,
plotters, and web-sys. Only `winapi` in this list is actually needed for
`wasmtime`-the-binary and only on Windows as well but for now this is
the state of things related to `cargo vendor`. If this becomes an issue
we could specifically remove the bulky contents of crates in the
`vendor` directory such as `v8` since it's only used for fuzzing.
Closes#3808
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* Review comments
