Provide environment variable configuration option

[FranzBusch]

Motivation

We are currently providing a config option to provide a path to the protoc binary. This works great if your protoc is always the same; however, for a lot of packages you want to build them on various systems and the path to protoc is always different. We really can't solve this with any config file.

Modification

This PR adds the option to set the PROTOC_PATH env variable and picks it up in the plugin to find the protoc binary.

Result

We can now dynamically pass the protoc path to the plugin.

[FranzBusch]

@thomasvl @tbkka Would love to get your opinion on this one. While the config option that I provided in the JSON file is good to have and solves most problems. Sometimes, that didn't really work if you needed your package to be developed cross platform. Furthermore, non-leaf packages couldn't use it at all.
I have been trying a few different things for the last days and was almost going to give up but then I came across an idea of passing an env variable to the plugin. The great thing is this works across command line and Xcode if you start Xcode in the right way.

The real fix is still in SPM however I think it is worth adding this env variable escape hatch.

[tbkka]

I'm fine with it.

[thomasvl]

Looking at the rendered docs, two of the subsections (###) are getting detected. Can you try an extra blank line before them so we can fix those in the same PR? You can preview it before we merge it at https://github.com/FranzBusch/swift-protobuf/blob/fb-env-variable-for-protoc/Sources/protoc-gen-swift/Docs.docc/spm-plugin.md.

[thomasvl]

I guess we need to pull this to main also.

[thomasvl]

#1283 is a cherry-pick of it over to main.