Fix build on Windows

- Force encoding to UTF-8
 - Fix npm not being found on the path
 - Test building common/web on OSX and Windows

buildSrc/src/main/kotlin/cc/tweaked/gradle/Node.kt

@@ -46,7 +46,7 @@ abstract class NpmInstall : DefaultTask() {
     @TaskAction
     fun install() {
         project.exec {
-            commandLine("npm", "ci")
+            commandLine(ProcessHelpers.getExecutable("npm"), "ci")
             workingDir = projectRoot.get().asFile
         }
     }
@@ -59,6 +59,6 @@ abstract class NpmInstall : DefaultTask() {
 abstract class NpxExecToDir : ExecToDir() {
     init {
         dependsOn(NpmInstall.TASK_NAME)
-        executable = "npx"
+        executable = ProcessHelpers.getExecutable("npx")
     }
 }