Make 'source-root' init input relative to github.workspace

In the previous commit, the default value of the input is ${{ github.workspace }}
which means that most uses of this input would probably prefix their paths with
${{ github.workspace }}, especially since actions/checkout's 'path' input
must be under ${{ github.workspace }}. Therefore, it doesn't make much sense for
this to be an absolute file path.

Instead, it's more intuitive to make this relative to the repository.

src/init-action.ts

@@ -1,3 +1,5 @@
+import * as path from "path";
+
 import * as core from "@actions/core";
 
 import {
@@ -201,11 +203,12 @@ async function run() {
     const codeqlRam = process.env["CODEQL_RAM"] || "6500";
     core.exportVariable("CODEQL_RAM", codeqlRam);
 
-    const tracerConfig = await runInit(
-      codeql,
-      config,
-      getRequiredInput("source-root")
+    const sourceRoot = path.join(
+      getRequiredEnvParam("GITHUB_WORKSPACE"),
+      getOptionalInput("source-root") || ""
     );
+
+    const tracerConfig = await runInit(codeql, config, sourceRoot);
     if (tracerConfig !== undefined) {
       for (const [key, value] of Object.entries(tracerConfig.env)) {
         core.exportVariable(key, value);