Use path.resolve instead of path.join for sourceRoot

Thanks to @aibaars, `path.resolve()` should will nicely handle more use-cases, namely absolute paths better than `path.join()`.

Co-authored-by: Arthur Baars <aibaars@github.com>

src/init-action.ts

@@ -203,7 +203,7 @@ async function run() {
     const codeqlRam = process.env["CODEQL_RAM"] || "6500";
     core.exportVariable("CODEQL_RAM", codeqlRam);
 
-    const sourceRoot = path.join(
+    const sourceRoot = path.resolve(
       getRequiredEnvParam("GITHUB_WORKSPACE"),
       getOptionalInput("source-root") || ""
     );