Provide environment variable configuration option

# 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.

Plugins/SwiftProtobufPlugin/plugin.swift

@@ -56,8 +56,13 @@ struct SwiftProtobufPlugin: BuildToolPlugin {
  // We need to find the path of protoc and protoc-gen-swift
  let protocPath: Path
  if let configuredProtocPath = configuration.protocPath {
+ // The user set the config path in the file. So let's take that
  protocPath = Path(configuredProtocPath)
+ } else if let environmentPath = ProcessInfo.processInfo.environment["PROTOC_PATH"] {
+ // The user set the env variable. So let's take that
+ protocPath = Path(environmentPath)
  } else {
+ // The user didn't set anything so let's try see if SPM can find a binary for us
  protocPath = try context.tool(named: "protoc").path
  }
  let protocGenSwiftPath = try context.tool(named: "protoc-gen-swift").path

Sources/protoc-gen-swift/Docs.docc/spm-plugin.md

@@ -22,7 +22,7 @@ There are multiple ways to do this. Some of the easiest are:
 1. If you are on MacOS, installing it via `brew install protoc`
 2. Download the binary from [Google's github repository](https://github.com/protocolbuffers/protobuf).
 
-### Adding the proto files to your target
+### Adding the proto files to your target
 
 Next, you need to add the `.proto` files for which you want to generate your Swift types to your target's
 source directory. You should also commit these files to your git repository since the generated types
@@ -56,7 +56,7 @@ let package = Package(
 
 ```
 
-### Configuring the plugin
+### Configuring the plugin
 
 Lastly, after you have added the `.proto` files and modified your `Package.swift` manifest, you can now
 configure the plugin to invoke the `protoc` compiler. This is done by adding a `swift-protobuf-config.json`
@@ -88,13 +88,25 @@ is generating Swift types for the `Bar.proto` file with the `public` visibility.
 
 ### Defining the path to the protoc binary
 
+
 The plugin needs to be able to invoke the `protoc` binary to generate the Swift types. 
-There are two ways how this can be achieved. First, by default, the package manager looks into
+There are three ways how this can be achieved. First, by default, the package manager looks into
 the `$PATH` to find binaries named `protoc`. This works immediately if you use `swift build` to build
 your package and `protoc` is installed in the `$PATH` (`brew` is adding it to your `$PATH` automatically).
 However, this doesn't work if you want to compile from Xcode since Xcode is not passed the `$PATH`.
-To still make this work from Xcode you can point the plugin to the concrete location of the `protoc`
-compiler by changing the configuration file like this:
+You have to options to set the path of `protoc` that the plugin is going to use. Either you can set
+an environment variable `PROTOC_PATH` that gets picked up by the plugin. Here are two example how you
+can set the variable so that it gets picked up:
+```
+// swift build
+env PROTOC_PATH=/opt/homebrew/bin/protoc swift build
+
+// To start Xcode (Xcode MUST NOT be running before invoking this)
+env PROTOC_PATH=/opt/homebrew/bin/protoc xed .
+```
+
+The other way to point the plugin to the concrete location of the `protoc`
+compiler is by changing the configuration file like this:
 
 ```json
 {
@@ -104,6 +116,7 @@ compiler by changing the configuration file like this:
 
 ```
 
-> Warning: This only solves the problem for leaf packages that are using the Swift package manager
-plugin since there you can point the package manager to the right binary. If your package is **NOT** a
-leaf package and should build with Xcode, we advise not to adopt the plugin yet!
+> Warning: The configuration file option only solves the problem for leaf packages that are using the Swift package manager
+plugin since there you can point the package manager to the right binary. The environment variable
+does solve the problem for transitive packages as well; however, it requires your users to set
+the variable now. In general we advise against adopting the plugin as a non-leaf package!