feat: musl variants

[YOU54F]

Create musl based variants of

• pact-protobuf-plugin

Tracking issue:- pact-foundation/roadmap#30

Follows on from pact-foundation/pact-reference#372

Will also require updates in pact-plugin-cli to

1. provide support to check for and download musl variants
2. provide musl variants of the exe

Additionally

• pins cross to 0.2.5 to ensure we build against the correct version of glib, issue from pact-ref

[YOU54F]

I've tested this against various forks of our pact projects (pact-php, pact-js-core/pact-js) and my hello_ffi project and it is working well under alpine.

Failures are due to not having relevant access to tokens to publish pacts

Sample builds are available on my fork release

https://github.com/YOU54F/pact-protobuf-plugin/releases/tag/v-0.3.13

[rholshausen]

I think for executables we should only publish musl variants. It does not make sense to have a glibc version and a musl version. The musl version will run on glibc distributions. In fact, this was the reason musl was created.

[YOU54F]

That makes total sense Ron, I should have actually tested that out xD

this PR will be re-approached to consider building only a musl variant for linux, and providing it with the name of the existing glibc based executable.

The resulting binary should work for all, and simplifies any musl or os detection requirements for linux