Remove percent-encoded chars in build plugin paths (#32)

### Motivation:

Xcode provides paths to executables as strings referencing environment
variables and then passes those strings to the build plugin as URLs.
This meant that when we converted these URLs to strings they
percent-encoded some characters and were no longer valid paths e.g.

```
/${BUILD_DIR}/${CONFIGURATION}/protoc-gen-swift
```
became
```
/$%7BBUILD_DIR%7D/$%7BCONFIGURATION%7D/protoc-gen-swift
```

### Modifications:

Have our utility function for accessing absolute paths as strings strip
percent-encoding.

### Result:

The build plugin works in Xcode

Plugins/PluginsShared/PluginUtils.swift

@@ -109,8 +109,10 @@ func constructProtocGenGRPCSwiftArguments(
 
 extension URL {
   /// Returns `URL.absoluteString` with the `file://` scheme prefix removed
+  ///
+  /// Note: This method also removes percent-encoded UTF-8 characters
   var absoluteStringNoScheme: String {
-    var absoluteString = self.absoluteString
+    var absoluteString = self.absoluteString.removingPercentEncoding ?? self.absoluteString
     absoluteString.trimPrefix("file://")
     return absoluteString
   }